Embedded Developer

In this, the last of the series of posts regarding implementing SPI Slave devices using the STM8S, we will look at building a module for the Netduino Go. This post builds upon the two previous posts:

• STM8S SPI Slave
  A simple SPI implementation using hardware SPI (single byte exchange).
• STM8S SPI Slave (Part 2)
  Software SPI implementation (using buffers to allow the exchange of data packets).

Here we will build upon the buffering and overlay the the Netduino Go 1.0 protocol in order to allow the two devices to communicate. We will also extend the STM8S application to add a simple function table to allow the simple addition of extra functionality to the system.

The makers of the Netduino Go, Secret Labs, have not formally released the GoBus 1.0 specification as a document. They have however release the source code to some of their modules and this can be found in the Wiki. The code found in the Wiki posts along with discussions on various forums has been used in the production of the code presented here. Credit for help is due to Secret Labs for releasing the code and also to the following Netduino forum members:

• Chris Walker
• CW2

These forum members have given assistance in one form or another over the past few months and without their help this post would not have been possible.

GoBus 1.0 Protocol

The early GoBus protocol uses an 18 byte data packet which is exchanged by the Netduino Go and the module. This packet of data contains a one byte header, 16 bytes of data and a one byte checksum with the data packets being exchanged over SPI. With the exception of the header and the checksum it appears that meaning of the data within the 16 byte payload is determined by the module developer.

I would also point the reader to the blog post A Developers Introduction to GoBus by Matt Isenhower on the Komodex System web site.

Enumeration

When the Netduino Go is first powered it will look at each of the Go Sockets in turn and attempt to interrogate the modules which are connected to the main board. It does this by sending out a packet with a single byte header 0xfe followed by 16 data bytes and a checksum. From experience, the data bytes are normally set to 0xff.

The module attached to the socket is then required to respond with a header byte of 0x2a followed by the 16 byte GUID of the module and the checksum byte.

The end result of this exchange is that the Netduino Go will have built up a list of modules attached to the main board along with the corresponding socket numbers.

This process then allows the .NET code to connect to a module using the GUID or the GUID and socket number. Successful connection is indicated by the blue LED by the side of the socket being illuminated. A failed connection normally results in an exception being thrown.

Data Transfer/Module Control

When the code running on the Netduino Go has successfully attached to the module on a socket it can start to control/communicate with the module. At this point it appears that the protocol uses the header 0x80 to indicate transfer between the module and the main board. So our data packets remain 18 bytes with the following format:

• 0x80 – Header
• 16 byte payload
• 1 byte CRC

It appears that the meaning of the 16 byte payload is determined by the module developer.

GPIO Interrupt

The protocol also allows for the use of a single GPIO. This can be used as a signalling line to let either side know that an action is pending. Convention appears to be to use this to allow the module to let the code on the main board know that there is some data/action pending.

A Simple Module

We will be creating a simple module to illustrate how the STM8S and the Netduino code work together. In order to use the least hardware possible the module will perform a simple calculation and return the result. Our module will need to perform the following:

• Enumerate using a GUID allowing the Netduino Go to detect the module
• Receive a number from the Netduino Go
• Perform a simple calculation and notify the Netduino Go that the result is ready.
• Send the result back to the Netduino Go when requested.

This simple module illustrates the key types of communication which may be required of a simple module. It is of course possible to use these to perform other tasks such as controlling a LED or receiving input from a button or keypad.

Netduino Go Module Driver

The Netduino Go code derived from the C# code published by Secret Labs in their Wiki. The major changes which have been made for this post are really concerned with improving clarity (adding comments at each stage to expand on the key points etc.).

Module ID

Modules are identified using a GUID. This ID allows the GoBus to connect to a module by scanning the bus for the specified ID. It also allows the Netduino Go to verify that when connecting to a module on a specific socket that the module is of the correct type. So the first thing we will need to do is obtain a new GUID. There are various ways in which we can do this and the simplest way to do this is to use the Create GUID menu option in Visual Studio. You can find this in the Tools menu.

Once you have your GUID you need to break this down into an array of bytes. You can then enter this in the both the Netduino code and the STM8S code. You will find the appropriate line in the file BasicModule.cs. The code looks something like this:

REMEMBER: It is critical that you generate your own GUID as each module type will need to have distinct ID.

Scanning down the file a little way you will find the two constructors for the class. One takes a socket and attempts to bind to the specified module on the requested socket. The other will attach to the first available module on the GoBus.

Initialise

This method is key to allowing the Netduino Go to connect to the module. The method binds to the module (assuming the IDs match) and retrieves a list of resources which the driver can use to communicate with the module. In this case, the SPI information and the pin used as an interrupt port. The remainder of the method configures the module driver to use these resources.

One key point to note is the use of the AutoResetEvent object. This is used to allow the interrupt handler to communicate the fact that an event has occurred to the methods we will write. This can be done in a manner which is non-blocking.

AddFive Method

This is the first of our methods implementing the functionality which our module will provide. In our case, this method actually implements the simple arithmetic we will be asking the module to perform. We will be sending a byte to the module, the module will add five to the number passed and then make this available to the Netduino Go. The code looks like this:

public byte AddFive(byte value)
{
	int retriesLeft = 10;
	bool responseReceived = false;

	_writeFrameBuffer[0] = GO_BUS10_COMMAND_RESPONSE;
	_writeFrameBuffer[1] = CMD_ADD_FIVE;
	_writeFrameBuffer[2] = value;
	WriteDataToModule();
	while (!responseReceived && (retriesLeft > 0))
	{
		//
		//  We have written the data to the module so wait for a maximum 
		//  of 5 milliseconds to see if the module responds with some 
		//  data for us.
		//
		responseReceived = _irqPortInterruptEvent.WaitOne(5, false);
		if ((responseReceived) && (_readFrameBuffer[1] == GO_BUS10_COMMAND_RESPONSE))
		{
			//
			//  Module has responded so extract the result.  Note we should really
			//  verify the checksum at this point.
			//
			_writeFrameBuffer[0] = GO_BUS10_COMMAND_RESPONSE;
			_writeFrameBuffer[1] = CMD_GET_RESULT;
			WriteDataToModule();
			return(_readFrameBuffer[2]);
		}
		else
		{
			//
			//  No response within the 5ms so lets make another attempt.
			//
			retriesLeft--;
			if (retriesLeft > 0)
			{
				WriteDataToModule();
			}
		}
	}
	throw new Exception("AddFive cannot communicate with the Basic GO! module");
}

The first thing this the method does is to set up the _writeFrameBuffer with the header, the command number and the data we will be sending. The data is then written to the module using SPI.

Next we will wait a while for the module to indicate via the GPIO pin that it has processed the data and the result is ready. As we shall see later, the module has already put the result in the transmission buffer ready for retrieval. This will have been performed before the interrupt was generated. The following line performs the non-blocking check to see if the interrupt has been generated:

responseReceived = _irqPortInterruptEvent.WaitOne(5, false);

responseReceived will be true if the interrupt has been generated and the C# module code has received the event.

The final task is to retrieve the result from the module by sending a retrieve command. This is performed by the following code:

_writeFrameBuffer[0] = GO_BUS10_COMMAND_RESPONSE;
_writeFrameBuffer[1] = CMD_GET_RESULT;
WriteDataToModule();
return(_readFrameBuffer[2]);

STM8S Module

Much of the code required here has already been covered in the previous post, STM8S SPI Slave (Part 2). The protocol uses a small buffer to allow messages to be transferred between the STM8S and the Netduino Go. In order to make this work as a Netduino Go module we need to overlay the GoBus protocol onto the message buffers and provide a mechanism for interpreting these messages. The mechanism we adopted is as follows:

• All messages will be restricted to 18 bytes (one byte header, 16 bytes payload, one byte CRC)
• The request header (from the Netduino to the module) will be 0x80 allowing a 16 byte payload
• The response header (from the module to the Netduino) will be 0x2a followed by 0x80. This restricts the return payload to 15 bytes.
• The final byte will be a CRC calculated on the header and the payload

The way in which the protocol has been implemented here also places a restriction upon on the application. Firstly, the module must receive a request as a full payload. Only then can the module respond. This is where the GPIO interrupt discussed earlier comes into play.

The final part of the problem is to work out how to dispatch the messages received by the module. To do this we will use a function table.

For the remainder of this article we will restrict ourselves to looking at the new functionality we will be adding on top of the previous post in order to allow the creation of a module.

Function Table

A function table in C is a simple list of function pointers. We will add to this by allowing a variable function identifier to be used to associate a byte ID with a particular method within the module. The following code allows the table to be setup:

//
//  Function table structure.
//
typedef struct
{
    unsigned char command;          //  Command number.
    void (*functionPointer)();      //  Pointer to the function to be executed.
} FunctionTableEntry;
//
//  Forward function declarations for the function table.
//
void AddFive();
void GetValue();
//
//  Table of pointers to functions which implement the specified commands.
//
FunctionTableEntry _functionTable[] = { { 0x01, AddFive }, { 0x02, GetValue } };
//
//  Number of functions in the function table.
//
const int _numberOfFunctions = sizeof(_functionTable) / sizeof(FunctionTableEntry);

Here we define a function table entry which has a byte ID and a pointer to a function (taking a void parameter list) associated with the ID. We then declare an array of these objects and associate functions with the IDs.

The final line of code simply determines the number of entries in the function table.

Using the above table we can work out which function to call using the following code:

if (_numberOfFunctions > 0)
{
	for (int index = 0; index < _numberOfFunctions; index++)
	{
		if (_functionTable[index].command == _rxBuffer[1])
		{
			(*(_functionTable[index].functionPointer))();
			break;
		}
	}
}

The function table method presented here allows the functionality of the module to be expanded with relative ease. In order to add a new piece of functionality you simply need to do the following:

• Create a new method in the STM8S code to implement the new functionality
• Determine the ID to be used for the functionality and add a new entry to the function table
• Create a method in the Netduino Go driver to call the method and retrieve any results as necessary

By performing these three simple steps you can add one or more functions with ease. The communication protocol will continue to work as is with no modification. The only exception to this rule will be cases where more than one payload of data needs to be transferred in order to achieve a specified piece of functionality (say a network driver etc.).

Buffers and GUIDs

We will need to make a slight modification to the Rx buffer in order to account for the checksum byte. We will also need to add somewhere to store the GUID which acts as the identifier for this module. This results in the following small change to the global variable code:

//
//  Application global variables.
//
unsigned char _rxBuffer[GO_BUFFER_SIZE + 1];    // Buffer holding the received data plus a CRC.
unsigned char _txBuffer[GO_BUFFER_SIZE];        // Buffer holding the data to send.
unsigned char *_rx;                             // Place to put the next byte received.
unsigned char *_tx;                             // Next byte to send.
int _rxCount;                                   // Number of characters received.
int _txCount;                                   // Number of characters sent.
volatile int _status;                           // Application status code.
//
//  GUID which identifies this module.
//
unsigned char _moduleID[] = { 0x80, 0x39, 0xe8, 0x2b, 0x55, 0x58, 0xeb, 0x48,
                              0xab, 0x9e, 0x48, 0xd3, 0xfd, 0xae, 0x8c, 0xee };

GoBus Interrupt

The discussion of the code on the Netduino Go driver (on the main board) mentioned the fact that the module can raise an interrupt to signal the fact that an operation has completed and that data is ready for retrieval. In order to do this we raise an interrupt on one of the pins when we have processed the data. This code is trivial:

//
//  Raise an interrupt to the GO! main board to indicate that there is some data
//  ready for collection.  The IRQ on the GO! board is configured as follows:
//
//  _irqPort = new InterruptPort((Cpu.Pin) socketGpioPin, false, Port.ResistorMode.PullUp,
//                               Port.InterruptMode.InterruptEdgeLow);
//
void NotifyGOBoard()
{
    PIN_GOBUS_INTERRUPT = 0;
    __no_operation();
    PIN_GOBUS_INTERRUPT = 1;
}

This method is simple and really just toggles which is connected to GPIO pin on the Netduino Go socket.

Adding Functionality to the Module

In our simple case we need to add two pieces of functionality, the ability to add five to a number and then to allow the caller to retrieve the result. This results in the following two methods:

//
//  GO! function 1 - add 5 to byte 2 in the Rx buffer and put the answer into the
//  Tx buffer.
//
void AddFive()
{
    _txBuffer[1] = _rxBuffer[2] + 5;
    NotifyGOBoard();
}

//--------------------------------------------------------------------------------
//
//  GO! Function 2 - return the Tx buffer back to the GO! board.
//
void GetValue()
{
    NotifyGOBoard();
}

SPI Go Frame

The implementation of the SPI processing here is interrupt driven. As such we will need to allow a method of synchronising the payloads we receive. This application will do this using the rising edge of the chip select signal which is generated by the Netduino Go main board. This allows us for cater for the many scenarios (synchronisation, underflow and overflow).

In the case of underflow and synchronisation, the chip select signal will rise before we have enough data. In this case we have either a corrupt packet or we have started to recei8ve data part way through the packet. In this case we cannot sensibly process the data so we should throw away the packet and wait for the next one.

An overflow situation can occur when the Netduino Go sends more than 18 bytes in one packet of data. In this case we should detect this and prevent the buffers from overflowing.

In order to allow for these cases we reset the Go frame pointers when the chip select signal changes from low to high:

//
//  This method resets SPI ready for the next transmission/reception of data
//  on the GO! bus.
//
//  Do not call this method whilst SPI is enabled as it will have no effect.
//
void ResetGoFrame()
{
    if (!SPI_CR1_SPE)
    {
        (void) SPI_DR;                          //  Reset any error conditions.
        (void) SPI_SR;
        SPI_DR = GO_FRAME_PREFIX;               //  First byte of the response.
        _txBuffer[0] = _moduleID[0];            //  Second byte in the response.
        //
        //  Now reset the buffer pointers and counters ready for data transfer.
        //
        _rx = _rxBuffer;
        _tx = _txBuffer;
        _rxCount = 0;
        _txCount = 0;
        //
        //  Note the documentation states this should be SPI_CR2_CRCEN
        //  but the header files have SPI_CR_CECEN defined.
        //
        SPI_CR2_CECEN = 0;                      //  Reset the CRC calculation.
        SPI_CR2_CRCNEXT = 0;
        SPI_CR2_CECEN = 1;
        SPI_SR_CRCERR = 0;
    }
}

As we shall see later, the end of the SPI transmission with result in one of the following cases:

• Too little data received correctly. The rising chip select line will reset the buffer pointers and the data will be discarded.
• The correct amount of data received. In this case the buffer will be processed correctly.
• Too much data is received. The excess data will be discarded to prevent a buffer overflow.

The ResetGoFrame method is key in ensuring that the buffers are reset at the end of the SPI transmission indicated by the rising chip select line.

SPI Tx/Rx Interrupt Handler

This method is responsible for ensuring that the data is transmitted and received correctly. It works in much the same way as the previous buffered SPI example. The main difference between this module and the previous example is what happens when the first byte of the data received is equal to 0xfe. In this case the Tx buffer pointer is moved to point to the module ID. This ensures that the Netduino Go receives the correct response to the enumeration request.

Connecting the Boards

The application code contains a number of #if statements to take into account the differing pin layouts of the microcontrollers used. The following have been tested so far:

• STM8S103F3 TSSOP20 on a breadboard
• STM8S Discovery

The Protomodule has also been wired up for one particular module but at the time of writing the definitions have not been added to the sources.

In order to connect the Netduino Go main board to a module in development you will probably need to purchase some form of breakout such as the Komodex breakout board (Straight connectors or 90-Degree connectors).

Connecting the two boards should be a simple case of ensuring that the SPI pins are connected MOSI to MOSI, MISO to MISO, CS to CS and Clock to Clock. In the case of the Discovery board I used PB0 for the CS line and for the STM8S103 setup I used the standard pin PA3.

Running the Code

Running the code should be a simple case of loading the STM8S code into the IAR development environment first and the deploying the code to the chip. Hot F5 to run the code.

Next, load the visual studio code and deploy this to the Netduino Go main board. Hit F5 to run the code.

The C# code running in Visual Studio should start to print some diagnostic information to the debug window. You should see a series of statements of the form Adding 5 to 6 to give 11. The 6 is the parameter which has been sent to the module for processing and the 11 is the result.

Observations

I have seen differing behaviours to the way in which the debugger in IAR works with the code in the module. Occasionally the debugger will actually prevent the module from enumerating. This will result in an exception in Visual Studio. To date I have only seen this behaviour with the STM8S103 setup. The STM8S Discovery board seems to work correctly. If you have problems with this then the only suggestion is to detach IAR from the board and rely upon diagnostic information being dumped to a logic analyser. You will note that the test application which runs on the Netduino Go has the instantiation of the module wrapped in a while loop and a try block. This allows the test code to make several attempts at creating a new module instance. This should not be necessary in the final production code as this has not yet failed in a none debug environment.

This code has been tested with the simple module example here and also with a temperature and humidity sensor. The application enumerated OK and has been soak tested in two environments over a period of hours. The code seems to be stable and works well with the Netduino Go.

I originally tried to be ambitious with the interrupt service routine dealing with the chip select line. This gave me code which was simpler but lead to a timing issue. As it stands at the moment, dropping the chip select line from high to low starts the SPI processing. The time between this happening and the first clock transition is only 3.25us as measured on my logic analyser. This means that all of the preparation must be completed in 3.25us.

If we look at the diagram below you can see the timings at the start of the SPI communication:



SPI Timing Diagram

The two markers 1 & 2 indicate the time we have between the start of the comms indicated by CS falling to the first clock pulse. The Status Code trace is a debugging signal generated by the application. The rising edge indicates when the first line of the interrupt service routine for the CS line starts and the falling edge indicates the point where we have completed enough processing to allow SPI to be enabled.

Conclusion

This post shows how we to create a Netduino Go module using a standard communication protocol. Additional module functionality can simply be added by adding to the function table.

As noted at the start, this article is the combination of information provided by Netduino community members along with the module code which can be found in the Wiki.

As usual, the source code for this application is available for download (STM8S Go Module and Netduino Go – Basic Module Driver).

Source Code Compatibility

System	Compatible?
STM8S103F3 (Breadboard)
Variable Lab Protomodule
STM8S Discovery

In the previous post we looked at exchanging single bytes using SPI with a Netduino Plus acting as the SPI master device and the STM8S acting as a slave device. The code presented suffered from a few deficiencies:

• We could only exchange one byte and that was mirrored back to the master device
• The mirroring assumed that a byte received meant the STM8S was ready to send a byte back to the Netduino

In this post we will deal with both of these issues and also look at a new problem which can arise, namely synchronisation.

The aim of the code we will be developing is to receive a buffer of data and at the same time send a different buffer of data back to the master device.

Hardware

The hardware we will be using is identical to the initial SPI post. We will be using a few more bits from the registers in order to allow the STM8S application to determine the action we should be taking.

SPR_SR_OVR – Overflow Indicator

This bit will be set when the chip detects an overflow condition. This can happen if the bus speed is too high and the data is arriving at a rate which is faster than the Interrupt Service Routine (ISR) can process it.

SPI_SR_RXNE – Receive Buffer Not Empty

This bit indicates that the receive buffer is not empty and that data is ready to be read.

SPI_SR_TXE – Transmit Buffer Empty

This indicates that the SPI transmit buffer is empty and ready to receive another byte of data.

Netduino Plus Software

The software running on the Netduino Plus requires a small modification to allow it to send a buffer of data rather than a single byte. We will also take the opportunity to increase the SPI bus speed to 500KHz. The code running on the Netduino Plus becomes:

public class Program
{
	/// <summary>
	/// SPI object.
	/// </summary>
	private static SPI spi = null;

	/// <summary>
	/// Configuration of the SPI port.
	/// </summary>
	private static SPI.Configuration config = null;

	public static void Main()
	{
		config = new SPI.Configuration(SPI_mod: SPI.SPI_module.SPI1,        // Which SPI module to use?
									   ChipSelect_Port: Pins.GPIO_PIN_D10,  // Chip select pin.
									   ChipSelect_ActiveState: false,       // Chip select is low when SPI is active.
									   ChipSelect_SetupTime: 0,
									   ChipSelect_HoldTime: 0,
									   Clock_IdleState: false,              // Clock is active low.
									   Clock_Edge: true,                    // Sample on the rising edge.
									   Clock_RateKHz: 500);
		spi = new SPI(config);

		byte[] buffer = new byte[17];
		for (byte index = 0; index < 17; index++)
		{
			buffer[index] = index;
		}
		while (true)
		{
			for (byte counter = 0; counter < 255; counter++)
			{
				buffer[0] = counter;
				spi.Write(buffer);
				Thread.Sleep(200);
			}
		}
	}
}

As you can see, much of the code is the same as that presented in the previous post. This application will now transmit a 17 byte buffer to the SPI slave device. The first byte in the buffer will be a sequence number which will cycle through the values 0 to 254. The remaining bytes in the buffer will remain unchanged.

STM8S SPI Slave

The main changes we will be making are in the application running on the STM8S. In this case we need to deal with the following additional issues:

• Possible overflows due to the increased speed of the SPI bus
• Treating the receive and transmit scenarios as distinct cases
• Buffer overflows

The first thing we are going to need is somewhere to store the data. Looking at the Netduino Code we have defined the buffer size as 17 bytes. The corresponding declaration in the STM8S code look like this:

//--------------------------------------------------------------------------------
//
//  Miscellaneous constants
//
#define BUFFER_SIZE             17

//--------------------------------------------------------------------------------
//
//  Application global variables.
//
unsigned char _rxBuffer[BUFFER_SIZE];       // Buffer holding the received data.
unsigned char _txBuffer[BUFFER_SIZE];       // Buffer holding the data to send.
unsigned char *_rx;                         // Place to put the next byte received.
unsigned char *_tx;                         // Next byte to send.
int _rxCount;                               // Number of characters received.
int _txCount;                               // Number of characters sent.

We will also need to provide a mechanism to reset the SPI buffer pointers back to a default state ready to receive data:

//--------------------------------------------------------------------------------
//
//  Reset the SPI buffers and pointers to their default values.
//
void ResetSPIBuffers()
{
    SPI_DR = 0xff;
    _rxCount = 0;
    _txCount = 0;
    _rx = _rxBuffer;
    _tx = _txBuffer;
}

We also no longer have a single byte of data to output on the diagnostic pins. We therefore need to add a new diagnostic method to output the data we are receiving.

//--------------------------------------------------------------------------------
//
//  Bit bang a buffer of data on the diagnostic pins.
//
void BitBangBuffer(unsigned char *buffer, int size)
{
    for (int index = 0; index < size; index++)
    {
        BitBang(buffer[index]);
    }
}

The main method needs to be modified to take into account the changes we have made. The code becomes:

int main(void)
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    InitialiseSPIAsSlave();
    ResetSPIBuffers();
    for (unsigned char index = 0; index < BUFFER_SIZE; index++)
    {
        _txBuffer[index] = index + 100;
    }
    InitialiseOutputPorts();
    _status = SC_UNKNOWN;
    __enable_interrupt();
    //
    //  Main program loop.
    //
    while (1)
    {
        __wait_for_interrupt();
        if (_status == SC_RX_BUFFER_FULL)
        {
            BitBangBuffer(_rxBuffer, BUFFER_SIZE);
        }
        _status = SC_UNKNOWN;
    }
}

So far all of the code changes have been to support the initialisation and configuration of the system. The one area we have not touched upon is processing of the data which is being transmitted / received, namely the SPI ISR.

SPI Interrupt Service Routine

For the application we have built so far, the ISR must take into account three possible scenarios:

• Buffer Overflow
• Data received
• Data transmission buffer empty

The code will utilise the three status we identified earlier in order to determine which action to take. In each case we will do the following:

• SPI Overflow (SPI_SR_OVR is set)
  Use the status codes to indicate an overflow has occurred and exit the ISR
• Data Received (SPI_SR_RXNE is set)
  Add the byte received to the buffer and update the buffer pointers. Set the status code to indicate that we have received some data.
• Data transmission buffer empty (SPI_SR_TXNE is set)
  Grab the next byte from the transmit buffer and send it. Update the transmit buffer pointers accordingly.

We will be adopting a naïve buffering solution for this application. The buffers will be circular. The ISR can assume that there is space to save the next byte (i.e. we never overflow) as when we reach the end of the buffer we simply set the pointer back to the start again. The code for the ISR becomes:

#pragma vector = SPI_TXE_vector
__interrupt void SPI_IRQHandler(void)
{
    //
    //  Check for an overflow error.
    //
    if (SPI_SR_OVR)
    {
        (void) SPI_DR;                      // These two reads clear the overflow
        (void) SPI_SR;                      // error.
        _status = SC_OVERFLOW;
        OutputStatusCode(_status);
        return;
    }
    //
    //  Looks like we have a valid transmit/receive interrupt.
    //
    if (SPI_SR_RXNE)
    {
        //
        //  We have received some data.
        //
        *_rx = SPI_DR;              //  Read the byte we have received.
        _rx++;
        _rxCount++;
        if (_rxCount == BUFFER_SIZE)
        {
            _status = SC_RX_BUFFER_FULL;
            OutputStatusCode(_status);
            _rx = _rxBuffer;
            _rxCount = 0;
        }
    }
    if (SPI_SR_TXE)
    {
        //
        //  The master is ready to receive another byte.
        //
        SPI_DR = *_tx;
        _tx++;
        _txCount++;
        if (_txCount == BUFFER_SIZE)
        {
            OutputStatusCode(SC_TX_BUFFER_EMPTY);
            _tx = _txBuffer;
            _txCount = 0;
        }
    }
}

If we run these two applications and connect the logic analyser we are likely to see traces similar to the following:



SPI Slave Buffered output on Logica Analyser

This is not what we expected. In fact we expect to see something like the following:



Correctly synchronised SPI buffered output on the Logic Analyser

The reason for this is the simple buffering and we have used and the fact that there we have not implemented a method for synchronising the two systems (Netduino and STM8S). The trace can be understood if we follow the deployment and startup cycles for each application. The sequence of events will proceed something like the following:

• Deploy code to the Netduino Plus
  At this point the application will start to run. We will be outputting a sequence of bytes followed by a 200ms pause.
• Deploy the code to the STM8S
  The application on the STM8S starts and waits for data to be received on the SPI bus.

It is highly possible that when the application on the STM8S starts we will be part way through the transmission of a sequence of bytes by the Netduino. Let us make the assumption that this is the case and the Netduino is transmitting byte 16.

• Byte 16 transmitted by Netduino
  The byte is received by the STM8S and put into the buffer at position 0. The buffer pointers are moved on to point to position 1.
• Byte 17 is transmitted by the Netduino
  The byte is received by the STM8S and put into the buffer at position 1. The buffer pointers are moved on to point to position 2.
• Netduino enters the 200ms pause
  The STM8S waits for the next byte
• Byte 0 transmitted by Netduino
  The byte is received by the STM8S and put into the buffer at position 2. The buffer pointers are moved on to point to position 3.

This sequence of events continues until the buffer on the STM8S is full. As you can see, the buffers started out unsynchronised and continue in this manner ad infinitum.

Interestingly, if you power down the two boards and then power them up simultaneously (or power up the STM8S and then the Netduino Plus) you will see the synchronised trace. This happens because the STM8S has been allowed to enter the receive mode before the Netduino Plus could start to send data.

Synchronising the Sender and Receiver

The key to the synchronisation is this case is to consider using an external signal to indicate the start of transmission of the first byte of the buffer. In theory this is what the NSS signal (chip select) is for. The STM8S does not provide a mechanism to detect the state change for the NSS line when operating in hardware mode (which is how the application has been operating so far). In order to resolve this we should consider converting the application to use software chip select mode.

Chip Select

The first thing to be considered is the port we will be using to detect the chip select signal. In this case we will be using Port B, pin 0. This port will need to be configured as an input with the interrupts enabled. The InitialisePorts method becomes:

void InitialisePorts()
{
    //
    //  Initialise Port D for debug output.
    //
    PD_ODR = 0;             //  All pins are turned off.
    PD_DDR = 0xff;          //  All pins are outputs.
    PD_CR1 = 0xff;          //  Push-Pull outputs.
    PD_CR2 = 0xff;          //  Output speeds upto 10 MHz.
    //
    //  Initialise Port B for input.
    //
    PB_ODR = 0;             //  Turn the outputs off.
    PB_DDR = 0;             //  All pins are inputs.
    PB_CR1 = 0xff;          //  All inputs have pull-ups enabled.
    PB_CR2 = 0xff;          //  Interrupts enabled on all pins.
    //
    //  Now set up the interrupt behaviour.
    //
    EXTI_CR1_PBIS = 2;      //  Port B interrupt on falling edge (initially).
}

One point to note about the above method is that we initially only detect the falling edge of the chip select signal. My first attempt at this code had both falling and rising edge detection in place. With this method enabled I found it difficult to detect which edge was causing the interrupt to be triggered. I therefore decided to initially detect only the falling edge. I would then add code to change the edge being detected to the ISR controlling the chip select. The code which detects the change of state for the chip select pin is as follows:

#pragma vector = 6
__interrupt void EXTI_PORTB_IRQHandler(void)
{
    if (EXTI_CR1_PBIS == 1)
    {
        //
        //  Transition from low to high disables SPI
        //
        SPI_CR1_SPE = 0;                        //  Disable SPI.
        SPI_CR2_SSI = 1;
        EXTI_CR1_PBIS = 2;                      //  Waiting for falling edge next.
        OutputStatusCode(SC_CS_RISING_EDGE);
    }
    else
    {
        //
        //  Transition from high to low selects this slave device.
        //
        EXTI_CR1_PBIS = 1;                      //  Waiting for rising edge next.
        ResetSPIBuffers();
        (void) SPI_DR;
        (void) SPI_SR;
        SPI_DR = *_tx++;                        //  Load the transmit with first byte.
        _txCount++;
        SPI_CR2_SSI = 0;
        SPI_CR1_MSTR = 0;
        SPI_CR1_SPE = 1;                        // Enable SPI.
        OutputStatusCode(SC_CS_FALLING_EDGE);
    }
}

This code performs two tasks:

• Falling Edge – Enable SPI
  Resets the SPI buffers and the SPI registers ready for data transmission> Next, enable SPI. Finally, setup the chip select to detect a rising edge.
• Rising Edge – Disable SPI
  Disables SPI and sets the chip select to look for a falling edge.

You will also note a few lines outputting status information. These should be removed in production code but are left in here in order to aid debugging.

The final thing we need to do is to modify the initialisation of the SPI registers. These are small changes and merely change the system from hardware to software chip select. One key change is that we do not enable SPI here. This is left to the chip select interrupt handler. The new version of the InitialiseSPIAsSlave method becomes:

void InitialiseSPIAsSlave()
{
    SPI_CR1_SPE = 0;                    //  Disable SPI.
    SPI_CR1_CPOL = 0;                   //  Clock is low when idle.
    SPI_CR1_CPHA = 0;                   //  Sample the data on the rising edge.
    SPI_ICR_TXIE = 1;                   //  Enable the SPI TXE interrupt.
    SPI_ICR_RXIE = 1;                   //  Enable the SPI RXE interrupt.
    SPI_CR2_SSI = 0;                    //  This is SPI slave device.
    SPI_CR2_SSM = 1;                    //  Slave management performed by software.
}

Conclusion

This post shows how we can overcome the naïve data transmission method presented by the previous post and add the ability to buffer data and to store a buffered response. Running the final version of the code overcomes the synchronisation problem we encountered at the expense of performing out own chip select handling in software.

As usual, the source code for this application is available for download (STM8S SPI Slave and Netduino SPI Master).

Source Code Compatibility

System	Compatible?
STM8S103F3 (Breadboard)
Variable Lab Protomodule
STM8S Discovery

For the next few posts I will be taking a look at SPI and how to use this to allow communication between two devices.

For some background reading I suggest that you visit Wikipedia and read the article on SPI. This post will assume that you are familiar with the material in that article.

In the first of the series we are going to be implementing a simple byte transfer between two devices, namely:

• Netduino Plus
• STM8S Discovery board

This scenario will require that SPI on the STM8S operates in slave mode as SPI on the Netduino Plus can only operate as a SPI master device. The Netduino family of products was chosen as the master because the SPI implementation is quick to setup and use. This means that we can be sure that any problems which arise during development are highly likely to be related to the STM8S code.

The problem definition is as follows:

• Configure the Netduino Plus as a SPI master device
• Send a repeated pattern of bytes over SPI to a listening slave device
• Configure the STM8S to operate as a SPI slave device
• Read the bytes from the SPI bus (MOSI) and send them back out on the bus (MISO)

We will also add some debugging code to allow us to connect a logic analyser to the STM8S and verify the data which is being received.

SPI Master – Netduino Plus Code

The initial version of this application will use a low bus speed for the SPI communication. By using a low speed we will reduce the influence of transmission errors and also allow the code to be debugged and logic errors eliminated without worrying too much about errors introduced through timing issues.

Our simple application looks like this:

public class Program
{
	/// <summary>
	/// SPI object.
	/// </summary>
	private static SPI spi = null;

	/// <summary>
	/// Configuration of the SPI port.
	/// </summary>
	private static SPI.Configuration config = null;

	public static void Main()
	{
		config = new SPI.Configuration(SPI_mod: SPI.SPI_module.SPI1,        // Which SPI module to use?
									   ChipSelect_Port: Pins.GPIO_PIN_D10,  // Chip select pin.
									   ChipSelect_ActiveState: false,       // Chip select is low when SPI is active.
									   ChipSelect_SetupTime: 0,
									   ChipSelect_HoldTime: 0,
									   Clock_IdleState: false,              // Clock is active low.
									   Clock_Edge: true,                    // Sample on the rising edge.
									   Clock_RateKHz: 10);
		spi = new SPI(config);

		byte[] buffer = new byte[1];
		while (true)
		{
			for (byte counter = 0; counter < 255; counter++)
			{
				buffer[0] = counter;
				spi.Write(buffer);
				Thread.Sleep(200);
			}
		}
	}
}

The configuration of the SPI bus is as follows:

• Chip select is digital pin 10
• Chip select is low when the bus is active
• Clock is active low (Clock Polarity – CPOL)
• Data will be valid on the rising clock edge (Clock Phase – CPHA)
• Clock frequency is 10Khz

It is important to note that the sampling settings must be duplicated on the slave device.

Once the SPI bus is configured, the application continuously loops outputting the bytes 0 to 254 on the SPI bus with a 200ms pause between each byte.

SPI Slave – STM8S

With the exception of the clock speed, we now need to configure the STM8S as a slave device using the same settings as the SPI master device.

The Registers

SPI_CR1_CPOL – Clock Polarity

The first setting we will consider is the clock polarity (CPOL). This is controlled by the CPOL bit in the CR1 register. This is defined as:

The master has an active low clock.

SPI_CR1_CPHA – Clock Phase

The clock phase determines when the data is ready to be sampled, i.e. on the rising or falling clock edge.

We will be sampling on the first clock transition, on the rising edge.

SPI_CR1_SPE – Enable or Disable SPI

This register determines if SPI is enabled or disabled. Setting this register to 0 disables SPI, setting it to 1 enables SPI.

SPI_ICR_TXIE and SPI_ICR_RXNE – Interrupt Enable/Disable

These two registers determine if the SPI interrupts will be triggered on transmit (TXIE) or receive (RXIE). Setting a bit to 0 will disable the interrupt, setting it to 1 will enable the interrupt.

SPI_DR – Data Register

The data register is used in two contexts, when data has been received and to transmit data. Reading this register will retrieve data from the receive buffer. Setting this register will load the specified value into the transmit buffer.

SPI_SR_RXNE – Receive Buffer Not Empty

This bit in the status register indicates if the receive buffer is empty. You can check this value before you read the SPI_DR register to determine if there is any data waiting to be read.

STM8S Code

The first thing we will need to do is to initialise the SPI bus matching the settings of the SPI master:

void InitialiseSPIAsSlave()
{
    SPI_CR1_SPE = 0;                    //  Disable SPI.
    SPI_CR1_CPOL = 0;                   //  Clock is low when idle.
    SPI_CR1_CPHA = 0;                   //  Sample the data on the rising edge.
    SPI_ICR_TXIE = 1;                   //  Enable the SPI TXE interrupt.
    SPI_ICR_RXIE = 1;                   //  Enable the SPI RXE interrupt.
    SPI_CR1_SPE = 1;                    //  Enable SPI.
}

This code not only matches the master settings but also enables the generation of interrupts for transmit empty and receive not empty. These interrupts are handled by the following code:

#pragma vector = SPI_TXE_vector
__interrupt void SPI_IRQHandler(void)
{
    if (SPI_SR_RXNE)
    {
        unsigned char byte;
        byte = SPI_DR;          //  Read the byte we have received.
        SPI_DR = byte;          //  Now transmit the byte.
        //
        //  Output some debug information.
        //
        OutputStatusCode(SC_OK);
        BitBang(byte);
    }
}

This method checks the SPI_SR_RXNE flag to determine if the receive buffer is not empty. If there is data ready for processing then the data is retrieved and then transmitted back to the master.

Note that this method offers a naive approach to sending and receiving data, something we will overcome in subsequent posts.

We have also provided two methods for debugging, one will output a status code; the second will output a single byte by bit banging the data using two pins on an output port. We will need to use these methods with care when we start to look at higher transmission speeds. The two debug methods need to operate at speeds which allow the interrupt service routine to complete before the next interrupt is ready to be generated.

Hardware Setup

The connections between the two devices are straight forward. The following pins should be connected:

In addition to the above connections between the two boards we have three pins defined for debugging/diagnostics.

Port D, pin 2 will be used to output a status code. The code will be output as a series of high/low transitions.

Port D, pins 4 (clock) and 5 (data) will output debug/diagnostic data. This will be output in a similar form to the SPI data being transmitted on the SPI bus. This form has been chosen as it allows a logic analyser to be used to interpret the data.

Results

If we connect the two devices and hook up a logic analyser we get output similar to the following:

Logic Analyser Output

The top four traces represent the data which is being transmitted by the Netduino and the STM8S on the SPI bus along with the clock and select control signals. The labelling on the traces indicate which signal is being shown.

The traces labelled 4 and 5 show the data which has been output from the BitBang method.

The final trace shows the status code.

If we read this trace from left to right we can see that the Netduino master output the byte 213 on MOSI. At the same time, the STM8S is sending the byte 212 back to the master on the MISO line. This difference of one is caused by the fact that the transmission from the STM8S is always one behind the transmission by the master to the slave.

Traces 4 and 5 confirm that the STM8S has in fact received the byte 213.

We see that the first thing that happens is that the transmission of data starts on the MOSI and MISO lines simultaneously. When the data transmission has completed we have in interrupt generated and the status code of 1 is output. Finally the application copies the data received onto the diagnostic output.

Conclusion

This application represents the first step on the road to building a faster application capable of transmitting and receiving greater amounts of data.

As usual, the source code for this application is available for download (STM8S SPI Slave and Netduino SPI Master).

Source Code Compatibility

System	Compatible?
STM8S103F3 (Breadboard)
Variable Lab Protomodule
STM8S Discovery

I have recently been discussing a problem running one of the examples in this series on the STM8S Discovery board. After what seems like an eternity the problem was finally traced to the channel I was using in one of the timer examples. It turns out that Timer 1, channel 3 is connected to the touch sensor on the STM8S Discovery board. This means that the code does not generate the expected output. Credit for discovering this goes to Netduino Forum members Fabien and Gutworks.

This discussion also highlighted the fact that these samples were being used on two common development platforms, namely the Variable Labs Protomodule and the STM8S Discovery board. I have therefore modified the samples in order to support these platforms (where possible) as well as the development platform I am using. I will also be adding a compatibility table at the end of each post in the series to show which platforms on which the code has been tested.

Source Code

The following shows the current status of the sample code for the first nine articles in this series:

• Simple GPIO
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• Configuring the System Clock on the STM8S
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• External Interrupts on the STM8S
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• Using the UART on the STM8S Microcontroller
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• Generating a Regular Pulse Using Timer 2
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• Generating a PWM signal on the STM8S
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• Single Pulse Generation with the STM8S
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• Timer 1 Counting modes
  Tested with: STM8S103F3, Protomodule, STM8S Discovery
• Single conversion ADC on the STM8S
  Tested with: STM8S103F3, Protomodule, STM8S Discovery

You can download the latest sources in a single zip file.

In making the changes to make the programs run on as many of the platforms as possible I also standardised the outputs to make them as compatible across the platforms where possible. So the following changes have been made:

• Port D pin 4 has been used where possible for all programs with a single output.
• Timer 1, Channel 3 has been changed to Timer 1, Channel 4 as the pin used for this output channel as Timer 1, Channel 3 is connected to the touch sensor on the STM8S Discovery board.
• The UART example uses UART1 on the STM8s130F3 and Protomodule but UART2 on the STM8S Discovery board.
• The ADC example uses AIN4 on the STM8S103F3 and STM8S Discovery board but AIN3 on the Protomodule.

Directory Layout

All of the projects use a similar directory structure. Let’s look at the first article in the series (Simple GPIO) as an example.

Unzip the file and navigate to the main directory, if you have used the default setting when extracting the files it should be 1 – Simple GPIO.

The main directory should contain three subdirectories (Discovery, Protomodule and STM8S103F3) and a single file (main.c).

main.c

This file contains the source code for this example and it is shared by all of the projects. Future examples may contain more files here in which case each file will also be a common file to all of the projects.

Discovery, Protomodule and STM8S130F3 Directories

These directories contain the workspaces and projects for each of the target platforms. They will also contain any code which is specific to that platform. At the time of writing the following platforms are supported:

• STM8S103F3 – STM8S103F3 TSSOP20 platform (my reference platform)
• Protomodule – Variable Labs Protomodule
• Discovery – STM8S Discovery board

In this post we will have a look at the Analog to Digital Converter (ADC) on the STM8S microcontroller. The number of ADCs available will depend upon the STM8S you are using. We will be using ADC1 which should be present on all STM8S microcontrollers.

In order to show how the ADC works we will be using the STM8S as a dimmer switch for an LED. This simple example will demonstrate how we can read an analog value and use a PWM signal to control the brightness of an LED.

In order to do this we will need the following:

• Potentiometer to provide an analog signal varying from 3.3V to GND.
• LED circuit from the External Interrupts in the STM8S posting

We will also be using Timer 1, Channel 4 to generate a PWM signal to control the brightness of the LED (see Generating PWM Signals using the STM8S.

The algorithm we will be using is as follows:

1. Configure the system
2. Read the value from the ADC
3. Set the PWM output based upon the analog reading
4. Pause for 1/10th second
5. Repeat from step 2

We will achieve this by using interrupts from the following resources:

• Timer 3 – Generates the PWM signal which will be used to control the LED
• Timer 2 – 1/10th second interrupt which triggers the ADC process
• ADC – Conversion is completed, adjust the PWM output

ADC Features

The ADC has several modes of operations. We will be using the simplest, namely single conversion mode. As the name suggests, this mode performs a conversion on a specific channel. We will also instruct the microcontroller to generate an interrupt once the conversion is complete.

Amongst the other features and modes on the STM8S are the following:

• Single scan mode – Perform a single conversion on a number of channels.
• Continuous and Buffered Continuous – Perform continuous conversions. New conversions start as soon as the current conversion has completed.
• Continuous Scan – Similar to the Single Scan but operating on a number of channels. Conversion restarts from channel 0 when the last channel has been converted.
• Watchdog – Set upper and lower limits for the conversion. An interrupt can be generated if a conversion is above the upper or below the lower values.
• External Trigger – An external trigger is used to start a conversion.

The conversion takes 14uS after a stabilisation period. Once the stabilisation is complete, readings are available without any further pauses.

The Registers

So let’s have a look at the registers we will be using in order to control the ADC:

• ADC_CR2_ALIGN – Data alignment
• ADC_CSR_CH – Channel selection
• ADC_DRH/L – Analog conversion result
• ADC_CR1_ADON – Turn ADC on / off, trigger conversion
• ADC_CR3_DBUF – Data buffer Availability
• ADC_CSR_EOCIE- Enable ADC interrupts
• ADC_CSR_EOC – End of Conversion

ADC_CR1_ADON – ADC On/Off

The ADON flag determines of the ADC is on or off. It also determines if a conversion has been triggered. Setting ADON to 0 turns the ADC off. Setting ADON to 1 the first time turns the ADC on. Setting this value a second (or subsequent time) starts a conversion.

ADC_CSR_CH – Channel Selection

The CH flag determines the channel which should be converted.

ADC_CR2_ALIGN – Data Alignment

The ALIGN flag determines the type of alignment in the result registers. We will be setting this to right align (set to 1) the data in the registers.

ADC_DRH/L – Conversion Result

This pair of registers holds the result of the conversion. The order the registers should be read is dependent upon the alignment of the data in the registers. For right aligned data we need to read the DRL before DRH.

ADC_CSR_EOCIE – Enable ADC Interrupts

Turn the ADC interrupts on / off.

ADC_CSR_EOC – End of Conversion

This bit is set by the hardware when the conversion has completed. It should be reset by the software in the Interrupt Service Routine (ISR).

Unused Registers

The application we are going to be writing is simple and only performs a conversion once every 1/10th second. This is plenty of time to perform a conversion and process the data before the next conversion starts. As such, we do not need to check the overrun register. This register indicates if the data generated in the continuous mode was overwritten before it was used.

Hardware

This post requires some additional hardware to be added to the circuit containing the STM8S:

• LED which is being controller through a transistor configured as a switch
• Potentiometer to provide an analog signal for conversion

LED Output

Use the LED Output circuit in the post External Interrupts on the STM8S. Connect the base of the transistor to the output of Timer 1, Channel 3.

Potentiometer

Connect the potentiometer so that one pin is connected to ground and one to 3.3V. The output (the wiper) should be connected to AIN4 on the STM8S. I used a 10K potentiometer for this example.

Software

As we have already noted, we will be driving the application by using interrupts. We will also use a few techniques/methods from previous posts. So let’s look at each of the elements we will be using.

Timer 2 – Start Conversions

Timer 2 is used to generate 10 interrupts per second. Each interrupt will trigger a new conversion. Setting up the timer should look familiar:

void SetupTimer2()
{
    TIM2_PSCR = 0x05;       //  Prescaler = 32.
    TIM2_ARRH = 0xc3;       //  High byte of 50,000.
    TIM2_ARRL = 0x50;       //  Low byte of 50,000.
    TIM2_IER_UIE = 1;       //  Enable the update interrupts.
    TIM2_CR1_CEN = 1;       //  Finally enable the timer.
}

The ISR is a simple method, it has only one main function, namely to start the conversion.

#pragma vector = TIM2_OVR_UIF_vector
__interrupt void TIM2_UPD_OVF_IRQHandler(void)
{
    PD_ODR_ODR5 = !PD_ODR_ODR5;        //  Indicate that the ADC has completed.

    ADC_CR1_ADON = 1;       //  Second write starts the conversion.

    TIM2_SR1_UIF = 0;       //  Reset the interrupt otherwise it will fire again straight away.
}

Note the comment on the ADC register Second write starts the conversion. This method assumes that we have set ADC_CR1_ADON at least once previously. As you will see later, we set this register in the setup method for the ADC.

In addition to the starting of the conversion, we have also added a line of code to toggle PD5. This will show us when the ISR has been triggered and is really only there for debugging.

Timer 1, Channel 4 – PWM Signal

The ADC generates a 10 bit value. We will therefore set up Timer 1 to generate a PWM signal which is 1024 (2¹⁰) clock signals in width. We can therefore use the value from the conversion to directly drive the PWM duty cycle.

void SetupTimer1()
{
    TIM1_ARRH = 0x03;       //  Reload counter = 1023 (10 bits)
    TIM1_ARRL = 0xff;
    TIM1_PSCRH = 0;         //  Prescalar = 0 (i.e. 1)
    TIM1_PSCRL = 0;
    TIM1_CR1_DIR = 1;       //  Down counter.
    TIM1_CR1_CMS = 0;       //  Edge aligned counter.
    TIM1_RCR = 0;           //  Repetition count.
    TIM1_CCMR4_OC4M = 7;    //  Set up to use PWM mode 2.
    TIM1_CCER2_CC4E = 1;    //  Output is enabled.
    TIM1_CCER2_CC4P = 0;    //  Active is defined as high.
    TIM1_CCR4H = 0x03;      //  Start with the PWM signal off.
    TIM1_CCR4L = 0xff;
    TIM1_BKR_MOE = 1;       //  Enable the main output.
    TIM1_CR1_CEN = 1;
}

Note that the Auto-reload registers is set to 1023 (0x3ff). We also set the capture compare registers to 1023 at the start. This will turn the LED off when the program starts.

System Clock

The program will be generating a PWM signal with a reasonably high clock frequency. In order to do this we will set the clock to use the internal oscillator running at 16MHz.

void InitialiseSystemClock()
{
    CLK_ICKR = 0;                       //  Reset the Internal Clock Register.
    CLK_ICKR_HSIEN = 1;                 //  Enable the HSI.
    CLK_ECKR = 0;                       //  Disable the external clock.
    while (CLK_ICKR_HSIRDY == 0);       //  Wait for the HSI to be ready for use.
    CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
    CLK_PCKENR1 = 0xff;                 //  Enable all peripheral clocks.
    CLK_PCKENR2 = 0xff;                 //  Ditto.
    CLK_CCOR = 0;                       //  Turn off CCO.
    CLK_HSITRIMR = 0;                   //  Turn off any HSIU trimming.
    CLK_SWIMCCR = 0;                    //  Set SWIM to run at clock / 2.
    CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
    CLK_SWCR = 0;                       //  Reset the clock switch control register.
    CLK_SWCR_SWEN = 1;                  //  Enable switching.
    while (CLK_SWCR_SWBSY != 0);        //  Pause while the clock switch is busy.
}

GPIO – Debug Signals

As with previous examples, we will configure some of the output ports so that we can generate debug signals:

void SetupOutputPorts()
{
    PD_ODR = 0;             //  All pins are turned off.
    //
    //  PD5 indicates when the ADC is triggered.
    //
    PD_DDR_DDR5 = 1;
    PD_CR1_C15 = 1;
    PD_CR2_C25 = 1;
    //
    //  PD4 indicated when the ADC has completed.
    //
    PD_DDR_DDR4 = 1;
    PD_CR1_C14 = 1;
    PD_CR2_C24 = 1;
}

ADC

The setup method for the ADC is relatively simple as many of the settings we will be using are the defaults after a reset. This method is as follows:

void SetupADC()
{
    ADC_CR1_ADON = 1;       //  Turn ADC on, note a second set is required to start the conversion.

#if defined PROTOMODULE
    ADC_CSR_CH = 0x03;      //  Protomodule uses STM8S105 - no AIN4.
#else
    ADC_CSR_CH = 0x04;      //  ADC on AIN4 only.
#endif

    ADC_CR3_DBUF = 0;
    ADC_CR2_ALIGN = 1;      //  Data is right aligned.
    ADC_CSR_EOCIE = 1;      //  Enable the interrupt after conversion completed.
}

After calling this method, the ADC should be powered on and ready to perform a conversion. Note that the ADC will not perform a conversion until ADC_CR1_ADON is set for a second time. This will be performed by the Timer 2 interrupt.

Another point to note is that on the Protomodule board the version of the STM8S does not have the AIN4 channel and so we use AIN3 instead.

The next method we will consider is the ADC ISR. This is where the real work of changing the values for the PWM signal takes place.

#pragma vector = ADC1_EOC_vector
__interrupt void ADC1_EOC_IRQHandler()
{
    unsigned char low, high;
    int reading;

    ADC_CR1_ADON = 0;       //  Disable the ADC.
    TIM1_CR1_CEN = 0;       //  Disable Timer 1.
    ADC_CSR_EOC = 0;        //     Indicate that ADC conversion is complete.

    low = ADC_DRL;            //    Extract the ADC reading.
    high = ADC_DRH;
    //
    //  Calculate the values for the capture compare register and restart Timer 1.
    //
    reading = 1023 - ((high * 256) + low);
    low = reading & 0xff;
    high = (reading >> 8) & 0xff;
    TIM1_CCR3H = high;      //  Reset the PWM counters.
    TIM1_CCR3L = low;
    TIM1_CR1_CEN = 1;       //  Restart Timer 1.

    PD_ODR_ODR4 = !PD_ODR_ODR4;     //  Indicate we have processed an ADC interrupt.
}

Note that we once again use a GPIO port to indicate when the ISR has been called.

Main Program Loop

The main program loop looks pretty much like the programs we have written in previous examples:

void main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    SetupTimer1();
    SetupTimer2();
    SetupOutputPorts();
    SetupADC();
    __enable_interrupt();
    while (1)
    {
        __wait_for_interrupt();
    }
}

Results

If we put all of this together we can do the following:

The LED indicates the duty cycle of the PWM signal. The output on the oscilloscope confirms the changes being made to the PWM signal (the wider the high component of the signal, the brighter the LED should be).

By adjusting the trimmer potentiometer to ground (turning to the right) the LED becomes dimmer as the duty cycle becomes biased towards ground (off more than on). Turning to the left does the reverse, the PWM signal becomes biased to +3.3V (more on than off).

Conclusion

This example may be trivial as we could have easily just connected the LED and the potentiometer together. However, it does show how we can take a reading from an ADC and change the output of the microcontroller based upon the value.

As always, the source code is available for download. This application is compatible with my reference platform, the Variable Labs Protomodule and the STM8S Discovery board.

Source Code Compatibility

System	Compatible?
STM8S103F3 (Breadboard)
Variable Lab Protomodule
STM8S Discovery

In this article we will continue to look at Timer 1, specifically:

• counting modes
• repetition counter
• update/overflow events

This article will assume some knowledge from the following two posts published previously:

• Generating a Regular Pulse Using Timer 2
• Generating PWM Signals on the STM8S

Unlike previous posts we will not be resetting the system clock but will instead leave this running using the default 2 MHz internal HSI oscillator.

Test Environment

In order to demonstrate the features of the timer I will be using my Saleae Logic Analyser as we will be observing events which occur many times a second. I have the following connections set up:

This set up will result in a series of charts from the Logic software which look like this:

Logic Analyser Output

The top portion of the display (labelled TIM2 Overflow) indicates when an overflow event has occurred on Timer 2. This timer is used to control the application. A change from low to high starts Timer 1 and the timer window for the observation runs to the next transition from high back to low. There is then a pause and then the whole cycle starts again.

The centre portion (labelled TIM1 PWM) shows the PWM output of Timer 1, channel 4. This is used to demonstrate what happens as we change the various values in the timer registers.

The lower portion (labelled TIM1 Overflow) shows when the Timer 1 overflow event occurs.

One thing that you can do to make the capture of these traces easier is to set a sample trigger on one of the traces. If you look at the trace labelled TIM2 Overflow you will see that one of the four square buttons is highlighted. This is showing that capture of data will begin when the logic analyser detects a signal which changes from low to high. You can make use of this by deploying and starting the application as follows:

1. Compile and deploy the application
2. Start data capture on the logic analyser
3. Start the application

When you click the Start button on the logic analyser (step 2) a window will appear which indicates that the software is monitoring the data looking for a trigger (in this case rising edge on PD5) before it will start to capture data.

The Registers

The example code we will use shows how we can change the properties of the output and also the frequency of the interrupts generated by using the following registers:

• TIM1_CR1_DIR – Counter Direction
• TIM1_RCR – Repetition Counter

TIM1_CR1_DIR – Counter Direction

This register determines the direction of the counter, either up from 0 to the auto-reload values (set to 0) or down from the auto-reload value (set to 1).

TIM1_RCR – Repetition Counter

This counter can be used to determine the frequency of the overflow interrupts. Normally (i.e. by setting this register to 0) an overflow/underflow interrupt is generated every time the counter is reloaded from the auto-reload registers. By setting this register to any number other than zero (i.e. n), the overflow/underflow interrupt will only be generated after n + 1 overflow/underflows have been detected.

Software

We will use the registers described above to generate a series of 5 pulses every 50 mS. The code to do this is as follows:

#if defined DISCOVERY
    #include <iostm8S105c6.h>
#elif defined PROTOMODULE
    #include <iostm8s103k3.h>
#else
    #include <iostm8s103f3.h>
#endif
#include <intrinsics.h>

//--------------------------------------------------------------------------------
//
//  Timer 2 Overflow handler.
//
#pragma vector = TIM2_OVR_UIF_vector
__interrupt void TIM2_UPD_OVF_IRQHandler(void)
{
    if (PD_ODR_ODR5 == 1)
    {
        TIM1_CR1_CEN = 0;
        PD_ODR_ODR5 = 0;
        PD_ODR_ODR4 = 0;
    }
    else
    {
        //
        //  Force Timer 1 to update without generating an interrupt.
        //  This is necessary to makes sure we start off with the correct
        //  number of PWM pulses for the first instance only.
        //
        TIM1_CR1_URS = 1;
        TIM1_EGR_UG = 1;
        //
        //  Reset the indicators.
        //
        PD_ODR_ODR5 = 1;
        PD_ODR_ODR4 = 1;
        //
        //  Enable Timer 1
        //
        TIM1_CR1_CEN = 1;           //  Start Timer 1.
    }
    TIM2_SR1_UIF = 0;               //  Reset the interrupt otherwise it will fire again straight away.
}

//--------------------------------------------------------------------------------
//
//  Timer 1 Overflow handler.
//
#pragma vector = TIM1_OVR_UIF_vector
__interrupt void TIM1_UPD_OVF_IRQHandler(void)
{
    PD_ODR_ODR4 = !PD_ODR_ODR4; //0;                //  Signal to the user that Timer 1 has stopped.
    TIM1_CR1_CEN = 0;               //  Stop Timer 1.
    TIM1_SR1_UIF = 0;               //  Reset the interrupt otherwise it will fire again straight away.
}

//--------------------------------------------------------------------------------
//
//  Set up Timer 1, channel 3 to output a single pulse lasting 240 uS.
//
void SetupTimer1()
{
    TIM1_ARRH = 0x03;       //  Reload counter = 960
    TIM1_ARRL = 0xc0;
    TIM1_PSCRH = 0;         //  Prescalar = 0 (i.e. 1)
    TIM1_PSCRL = 0;
    //
    //  Select 0 for up counting or 1 for down counting.
    //
    TIM1_CR1_DIR = 0;       //  Up counter.
    //
    //  Select 0 for edge aligned, 1 for mode 1 centre aligned,
    //  2 for mode 2 centre aligned or 3 for mode 3 centre aligned.
    //
    TIM1_CR1_CMS = 0;       //  Edge aligned counter.
    //
    //  Set the following depending upon the number of PWM pulses, note
    //  n + 1 pulses will be generated before the interrupt.
    //
    TIM1_RCR = 4;           //  Repetition count.
    //
    //  Now configure Timer 1, channel 4.
    //
    TIM1_CCMR4_OC4M = 7;    //  Set up to use PWM mode 2.
    TIM1_CCER2_CC4E = 1;    //  Output is enabled.
    TIM1_CCER2_CC4P = 0;    //  Active is defined as high.
    TIM1_CCR4H = 0x01;      //  480 = 50% duty cycle (based on TIM1_ARR).
    TIM1_CCR4L = 0xe0;
    TIM1_BKR_MOE = 1;       //  Enable the main output.
    TIM1_IER_UIE = 1;       //  Turn interrupts on.
}

//--------------------------------------------------------------------------------
//
//  Setup Timer 2 to generate a 40 Hz interrupt based upon a 2 MHz timer.  This
//	will result in a signal with a frequency of 20Hz.
//
void SetupTimer2()
{
    TIM2_PSCR = 0x00;       //  Prescaler = 1.
    TIM2_ARRH = 0xc3;       //  High byte of 50,000.
    TIM2_ARRL = 0x50;       //  Low byte of 50,000.
    TIM2_IER_UIE = 1;       //  Enable the update interrupts.
    TIM2_CR1_CEN = 1;       //  Finally enable the timer.
}

//--------------------------------------------------------------------------------
//
//  Now set up the output ports.
//
//  Setup the port used to signal to the outside world that a timer event has
//  been generated.
//
void SetupOutputPorts()
{
    PD_ODR = 0;             //  All pins are turned off.
    //
    //  PD5 is used to indicate the firing of the update/overflow event for Timer 2
    //
    PD_DDR_DDR5 = 1;
    PD_CR1_C15 = 1;
    PD_CR2_C25 = 1;
    //
    //  PD4 is used to indicate the firing of the update/overflow event for Timer 1
    //
    PD_DDR_DDR4 = 1;
    PD_CR1_C14 = 1;
    PD_CR2_C24 = 1;
}

//--------------------------------------------------------------------------------
//
//  Main program loop.
//
void main()
{
    __disable_interrupt();
    SetupTimer1();
    SetupTimer2();
    SetupOutputPorts();
    __enable_interrupt();
    while (1)
    {
        __wait_for_interrupt();
    }
}

If we now have a look at the output on the Logic Analyser we can see how the various signals indicate what is happening with the Timers.

Five Pulses

The top trace of the output shows us when Timer 2 has overflowed (i.e. every 25mS). The code above goes through a cycle of enabling Timer 1 and 25mS later disabling Timer 1. You can see the output from Timer 1, Channel 3 in the middle of the three traces. The final (bottom) trace shows when Timer 1 overflows.

We have used much of the code presented above in previous examples in the series. We are setting up the two timers (Timer 1 and Timer 2, Channel 3) and adding interrupt handlers for the counter overflows. We are also setting up Port D as an output port to give us some diagnostic traces.

One difference from previous examples is that we are running this code at a slower clock frequency. By not setting up the system clock we are running at the default clock rate (2 MHz clock from the internal clock source).

Timer 1 Overflow Handler

This handler is really simple and does nothing more than use PD4 to indicate it has been called and then turns the timer off.

Timer 2 Overflow Handler

This handler is a little more complex. The first things to note is that it uses PD5 and works out if we are starting timers or pausing (PD5 = 1).

The next thing to note is the use of two more registers TIM1_CR1_URS and TIM1_EGR_UG. These two registers are used together to force the counter register to be reloaded without generating an interrupt. This is necessary to ensure that we start from a known value.

Conclusion

We have seen how we can use the logic analyser and some output pins (PD4 and PD5) to give us information about the sequence of events (interrupts). This is a very useful diagnostic tool and often the only one which is available to you when operating in this sort of environment.

I would also suggest trying some of the following and viewing the output on a logic analyser:

• Changing the counting mode
• Removing the update of TIM1_CR1_URS and/or TIM1_EGR_UG

As always, the source code is available for download.

Source Code Compatibility

System	Compatible?
STM8S103F3 (Breadboard)
Variable Lab Protomodule
STM8S Discovery

I have recently been looking at using a sensor which uses a one-wire communication protocol. The protocol uses a single pulse of a defined length to trigger the unit to send the sensor reading back down the same wire. This lead me on to thinking about how I could achieve this, the results of which are documented here.

Whilst the main purpose of the code we will be developing in this post remains the same, i.e. to produce a single pulse of a defined length, I felt it important to show the two fundamental ways in which this can be achieved:

• Interrupts and GPIO
• Timers

Much of the first method, using interrupts and GPIO signals is a relatively straight forward case of modifying one of the previous examples, namely Using Timers on the STM8S.

The second method is more interesting as we look at using Timers to solve this problem. This will start us looking at using Timer 1. This is probably the most flexible and powerful of the Timers on the STM8S. This power and flexibility comes with a price, it is also the most complex of the timers we have at our disposal.

As an aside, we will look at measuring the length of the pulses we can generate with the aim of defining the minimum pulse length we can create using each of the methods.

So let’s start with a common problem definition. We will use both methods to generate a single pulse lasting 30 uS.

Method 1 – Interrupts and GPIO

This method requires only slight modifications to the code presented in Using Timers on the STM8S. So let’s start by downloading the example and modifying the code.

The first thing we will need to do is to modify the duration of the timer in order to generate and interrupt every 30 uS. In the original program we setup Timer 2 as follows:

//
//  Setup Timer 2 to generate a 20 Hz interrupt based upon a 16 MHz timer.
//
void SetupTimer2()
{
    TIM2_PSCR = 0x03;       //  Prescaler = 8.
    TIM2_ARRH = 0xc3;       //  High byte of 50,000.
    TIM2_ARRL = 0x50;       //  Low byte of 50,000.
    TIM2_IER_UIE = 1;       //  Enable the update interrupts.
    TIM2_CR1_CEN = 1;       //  Finally enable the timer.
}

From the previous article we know that the following formula applies:

(2^TIM2_PSCR * counter) = f_master / f_interrupt

Now we are looking at generating a high frequency (low duration) pulse and so it is not unreasonable to set the prescalar to 1 (i.e. TIM2_PSCR = 0). This simplifies the formula to:

counter = f_master / f_interrupt

We also know that f_interrupt is given by the following formula:

f_interrupt = 1 / pulse duration

Putting the two together gives:

counter = f_master * pulse duration

counter = 16,000,000 * 30 * 10^-6

counter = 480 (0x1e0)

So our code becomes:

//
//  Setup Timer 2 to generate an interrupt every 480 clock ticks (30 uS).
//
void SetupTimer2()
{
    TIM2_PSCR = 0x00;       //  Prescaler = 1.
    TIM2_ARRH = 0x01;       //  High byte of 480.
    TIM2_ARRL = 0xe0;       //  Low byte of 480.
    TIM2_IER_UIE = 1;       //  Turn on the interrupts.
    TIM2_CR1_CEN = 1;       //  Finally enable the timer.
}

If you hook up oscilloscope and deploy the code you should find that the STM8S is generating square wave on Post D, Pin 5. The frequency of the signal should be 60 uS (see the previous article for an explanation where this comes from) with a duty cycle of 50%. Each of the components should have a width of 30 uS.

The next thing we need to do is to make the system generate a single pulse instead of a square wave. The solution is shockingly simple; in this case we turn off the timer interrupt after the first pulse has been generated.

The code in the Interrupt Service Routine (ISR) currently looks like this:

//
//  Timer 2 Overflow handler.
//
#pragma vector = TIM2_OVR_UIF_vector
__interrupt void TIM2_UPD_OVF_IRQHandler(void)
{
    PD_ODR_ODR4 = !PD_ODR_ODR4;     //  Toggle Port D, pin 4.
    TIM2_SR1_UIF = 0;               //  Reset the interrupt otherwise it will fire again straight away.
}

When we initialise the GPIO port we start with the output set to low. The timer interrupt code then toggles the GPIO port. So the first time this ISR is called the GPIO port goes high, the second time the GPIO port goes low etc. This means we need to turn off the interrupt when we transition from high to low for the first time. This results in the following code:

//
//  Timer 2 Overflow handler.
//
#pragma vector = TIM2_OVR_UIF_vector
__interrupt void TIM2_UPD_OVF_IRQHandler(void)
{
    unsigned char data;

    data = PD_ODR_ODR4;
    PD_ODR_ODR4 = !data;            //  Toggle Port D, pin 5.
    if (data == 1)
    {
        TIM2_IER_UIE = 0;           //  Only allow the pulse to happen once.
    }
    TIM2_SR1_UIF = 0;               //  Reset the interrupt otherwise it will fire again straight away.
}

This method turns off the Timer 2 interrupt and only the Timer 2 interrupt but resetting the interrupt enable flag for Timer 2 (TIM2_IER_UIE = 0). We could have called __disable_interrupt() here but this would have turned off all interrupts.

Deploying this code results in the following output on the oscilloscope:

And just to prove that the application generated a single pulse I hooked up the logic analyser and set this up to capture over 10 seconds worth of data. This resulted in the following output:

As you can see, we have what looks like a single pulse (see the logic analyser output). Zooming in on the pulse on the logic analyser output confirmed that there is indeed only a single pulse. A quick check of the oscilloscope output confirmed that the duration of the pulse is 30 uS.

Method 2 – Timers and PWM

In Generating PWM Signals using the STM8S we saw how we can generate a PWM signal without having to use interrupts. Here we will extend the principle to generating a single pulse using the One Pulse Mode (OPM) feature of Timer 1 (note that OPM is not available on Timer 2). As with the above, we will do this is two stages, namely to generate a PWM signal and then to restrict the output to a single pulse.

So let’s start by looking at Timer 1 and what we will need for this example.

TIM1_ARRH & TIM1_ARRL – Timer 1 Auto Reload Registers

As with Timer 2, these are two 8-bit registers which when combined make up the 16-bit counter value. To reset the 16-bit value we need to write to TIM1_ARRH before writing to TIM_ARRL as writing to TIM1_ARRL triggers the update of the registers.

TIM1_PSCRH & TIM1_PSCRL – Timer 1 Prescalar

This is a 16-bit register and allows finer control over the prescalar than we had with Timer 2. In this case the value can be any value from 0 to 65535. The frequency of the counter (f_counter) is given by the following frequency:

f_counter = f_master / (Prescalar + 1)

This means that the range of the divisor used is actually 1 to 65536.

As with the auto-reload register, we should load the high bits before the low bits (i.e. TIM1_PSCRH before TIM1_PSCRL).

TIM1_RCR – Timer 1 Repetition Counter

The repetition counter allows for the timer to generate update events only when a number of repetitions of the counter underflow and overflow have occurred. This is a topic which is outside of the scope of this example and so we will set this to 0 for the moment and return to this topic in future examples.

TIM1_CR1 – Timer 1 Control Register 1

We will be ensuring that two bits in this register are set; namely TIM1_CR1_DIR and TIM1_CR1_CMS.

TIM1_CR1_DIR controls the direction of the counter as counter 1 can count from 0 upwards or from TIM_ARR down to 0. Setting this value to 0 means count upwards whilst 1 means count downwards.

TIM1_CR1_CMS determines the counter alignment. For this example we will be using edge aligned counting and will be setting this to 0. Note that this value is a two bit value and the meaning of the remaining values is left for a future discussion.

TIM1_CCRM4 – Timer 1 Capture/Compare Mode Register 4

As with Timer 2, we can control the PWM mode setting this to either mode 1 or mode 2. We will configure this channel to be operating in PWM mode 2. In this mode OC3 will be inactive as long as the counter < TIM1_CCR3.

TIM1_CCR4H & TIM1_CCR4L – Timer 1 Capture Compare Register 4

These registers together form a 16-bit value for use in Capture/Compare/PWM mode. In PWM mode, these values coupled with the TIM1_ARR registers will allow control of the duty cycle of a PWM signal.

TIM1_CCER2 – Timer 1 Capture/Compare Register 2

This register determines the output polarity and availability of Timer 1, channel 4 (amongst other things). The bits we are really interested in are the availability and the polarity of the output.

TIM1_CCER2_CC4E determines if the output is enabled or disabled; 0 is disabled, 1 is enabled.

TIM2_CCER2_CC4P determines the polarity of the active stage of the output. A polarity of 0 means that the active stage gives a high (logic 1) output, whilst a polarity of 1 gives a low (logic 0) output.

Software

So if we put all of this together we get an application which looks something like this:

//
//  This program shows how you can generate a single pulse using
//  timers on the STM8S microcontroller.
//
//  This software is provided under the CC BY-SA 3.0 licence.  A
//  copy of this licence can be found at:
//
//  http://creativecommons.org/licenses/by-sa/3.0/legalcode
//
#if defined DISCOVERY
    #include <iostm8S105c6.h>
#elif defined PROTOMODULE
    #include <iostm8s103k3.h>
#else
    #include <iostm8s103f3.h>
#endif
#include <intrinsics.h>

//
//  Setup the system clock to run at 16MHz using the internal oscillator.
//
void InitialiseSystemClock()
{
    CLK_ICKR = 0;                       //  Reset the Internal Clock Register.
    CLK_ICKR_HSIEN = 1;                 //  Enable the HSI.
    CLK_ECKR = 0;                       //  Disable the external clock.
    while (CLK_ICKR_HSIRDY == 0);       //  Wait for the HSI to be ready for use.
    CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
    CLK_PCKENR1 = 0xff;                 //  Enable all peripheral clocks.
    CLK_PCKENR2 = 0xff;                 //  Ditto.
    CLK_CCOR = 0;                       //  Turn off CCO.
    CLK_HSITRIMR = 0;                   //  Turn off any HSIU trimming.
    CLK_SWIMCCR = 0;                    //  Set SWIM to run at clock / 2.
    CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
    CLK_SWCR = 0;                       //  Reset the clock switch control register.
    CLK_SWCR_SWEN = 1;                  //  Enable switching.
    while (CLK_SWCR_SWBSY != 0);        //  Pause while the clock switch is busy.
}

//
//  Set up Timer 1, channel 4 to output a single pulse lasting 30 uS.
//
void SetupTimer1()
{
    TIM1_ARRH = 0x03;       //  Reload counter = 960
    TIM1_ARRL = 0xc0;
    TIM1_PSCRH = 0;         //  Prescalar = 0 (i.e. 1)
    TIM1_PSCRL = 0;
    TIM1_CR1_DIR = 0;       //  Up counter.
    TIM1_CR1_CMS = 0;       //  Edge aligned counter.
    TIM1_RCR = 0;           //  No repetition.
    //
    //  Now configure Timer 1, channel 4.
    //
    TIM1_CCMR4_OC4M = 7;    //  Set up to use PWM mode 2.
    TIM1_CCER2_CC4E = 1;    //  Output is enabled.
    TIM1_CCER2_CC4P = 0;    //  Active is defined as high.
    TIM1_CCR4H = 0x01;      //  480 = 50% duty cycle (based on TIM1_ARR).
    TIM1_CCR4L = 0xe0;
    TIM1_BKR_MOE = 1;       //  Enable the main output.
	//
	//	Uncomment the following line to produce a single pulse.
	//
//    TIM1_CR1_OPM = 1;
    TIM1_CR1_CEN = 1;
}

//
//  Main program loop.
//
void main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    SetupTimer1();
    __enable_interrupt();
    while (1)
    {
        __wait_for_interrupt();
    }
}

If we run this application and hook up the oscilloscope to Timer 1, channel 4 (Pin 13 on the STM8S103F3 TSSOP20 package) we should find we get a PWM signal with a 60 uS period and a 50% duty cycle.

Timers and One Pulse Mode

Now that we have PWM functioning as expected we really only have to make one minor code modification, namely to set the timer generating a single pulse. For this we only need to add one line of code to the above application, SetupTimer1 becomes:

//
//  Set up Timer 1, channel 4 to output a single pulse lasting 30 uS.
//
void SetupTimer1()
{
    TIM1_ARRH = 0x03;       //  Reload counter = 960
    TIM1_ARRL = 0xc0;
    TIM1_PSCRH = 0;         //  Prescalar = 0 (i.e. 1)
    TIM1_PSCRL = 0;
    TIM1_CR1_DIR = 0;       //  Up counter.
    TIM1_CR1_CMS = 0;       //  Edge aligned counter.
    TIM1_RCR = 0;           //  No repetition.
    //
    //  Now configure Timer 1, channel 4.
    //
    TIM1_CCMR3_OC3M = 7;    //  Set up to use PWM mode 2.
    TIM1_CCER2_CC3E = 1;    //  Output is enabled.
    TIM1_CCER2_CC3P = 1;    //  Active is defined as high.
    TIM1_CCR3H = 0x01;      //  480 = 50% duty cycle (based on TIM1_ARR).
    TIM1_CCR3L = 0xe0;
    TIM1_BKR_MOE = 1;       //  Enable the main output.
    TIM1_CR1_OPM = 1;		//	Enable single pulse mode.
    TIM1_CR1_CEN = 1;
}

How Fast Can We Go?

Each of the above programs has their limitations. Assuming the same clock speed, the interrupt method is restricted by the number of instructions which must be executed in order to toggle the GPIO pin and work out if this is the first or seconded invocation of the ISR. The second is really only restricted by the speed of the system clock. This does not mean we cannot experiment to determine which is faster.

In both cases the programs were modified changing the timer auto-reload registers and the capture compare registers. The auto-reload register was always set to a value twice that of the capture compare register. For the interrupt method the fastest pulse which could be achieved was in the order of 2.5 uS (ARR = 0x0004) whilst the OPM method resulted in a pulse width of 146 nS (TIM1_ARR = 0x0002).

Conclusion

In this article we have looked at two methods which we can use to generate a single pulse. I am not offering advice on which is better, I’ll leave this to you as the application developer to decide.

Hopefully you will have gained an appreciation of the power of Timer 1. You should also have realised that using Timer 1 is not as simple as using Timer 2. There are a number of features we have not touched upon including (but not restricted to):

• Capture/Compare
• PWM Modes
• Timer synchronisation

I am sure that we shall return to Timer 1 in future posts.

As always, the source code is available for download.

Source Code Compatibility

System	Compatible?
STM8S103F3 (Breadboard)
Variable Lab Protomodule
STM8S Discovery

A while ago I wrote about using interrupts on the STM8S (see External Interrupts on the STM8S and hinted there that I would come back to the topic and here we are. In this article we will cover the following:

• List of interrupts available and the interrupt table
• Writing your own Interrupt Service Routine (ISR)
• Setting/Changing interrupt priorities

It is probably a good time to remind you that the STM8S Reference Manual (document number RM0016), available from ST’s web site is handy, to keep available for reference. This post is really meant for the application developer who wants to know which interrupts are available and how to use them.

Interrupt Table

The STM8S holds a list of interrupt vectors in a table in memory. As a programmer you are able to add your own ISRs to your application. These are really just conventional methods with a slightly different entry and exit mechanism. You don’t have to worry about how this mechanism works as you can make the compiler do all of the work for you. We shall see how later. The compiler is also be good enough to ensure that the table of interrupt vectors is also updated to point to your code as part of the application start up.

The following table lists the interrupts which are available on the STM8S103F3 microcontroller:

One thing to remember is that while the interrupt vector numbers remain unchanged the availability of the interrupt vectors will change depending upon the chip you are using. For instance, vector 10 is not used here but on the STM8S208 this is available to process one of the CAN interrupts. The simplest way to find out if an interrupt is available is to look at the header file for your chip. So let’s

So how do you know which interrupts are available for your microcontroller?

The first thing to note is that the vector numbers for interrupts 1-9 are usually the same as these features are available on most of the controllers. Note that whilst interrupt 9 is not available on my chip it is the EXTI_PORTE vector. I have not been able to locate any standard definitions (i.e. #define’s etc) for these interrupt vectors in any of the header files supplied with the compiler.

For the rest of the vectors we will have to start to look through the header files for the microcontroller. Opening up <iostm8s103f3.h> and going to the very end of the file we find a section with the comment Interrupt vector numbers. There should be one or more definitions for each of the features which allow the use of interrupts and which are available on the microcontroller.

One of the things to note about the list of available interrupts is that there are more than one #define statements for each feature. Consider the following extract:

/*-------------------------------------------------------------------------
 *      Interrupt vector numbers
 *-----------------------------------------------------------------------*/
#define SPI_TXE_vector                       0x0C
#define SPI_RXNE_vector                      0x0C
#define SPI_WKUP_vector                      0x0C
#define SPI_MODF_vector                      0x0C
#define SPI_CRCERR_vector                    0x0C
#define SPI_OVR_vector                       0x0C
#define TIM1_CAPCOM_TIF_vector               0x0D
#define TIM1_CAPCOM_BIF_vector               0x0D
#define TIM1_OVR_UIF_vector                  0x0D

If we look at the SPI definitions we can see that all of the vectors map to the same interrupt number. This is because there is only one ISR for this (SPI) feature. So the important point to take away is that your ISR must work out which condition it is working with.

Consider the example from the earlier post External Interrupts on the STM8S. This used the following ISR:

#pragma vector = 8
__interrupt void EXTI_PORTD_IRQHandler(void)
{
    PD_ODR_ODR3 = !PD_ODR_ODR3;     //  Toggle Port D, pin 3.
}

This method performed the same action every time this ISR was called. Now this did not matter for the example as we only had one switch attached to Port D. If we had another switch and LED attached to the same port then we would have had to work out which switch had been pressed in order to work out which action to take. Whilst this is simple in the case of switches the same principle applies to other features like SPI. In the case of SPI, the application should interrogate the status registers in order to work out why the ISR has been called.

Writing your own Interrupt Service Routine (ISR)

Writing your own ISR is really no different from writing any other method in C. There are a few extra rules which need to be followed but the majority of the techniques are the same. So let us return to the external interrupt code example:

#pragma vector = 8
__interrupt void EXTI_PORTD_IRQHandler(void)
{
    PD_ODR_ODR3 = !PD_ODR_ODR3;     //  Toggle Port D, pin 3.
}

The first thing you notice is the addition of the #pragma vector = 8 statement. This tells the compiler which interrupt vector we are going to be writing. In this case it is vector 8 which is the EXTI_PORTD interrupt vector (see the table above). You can also use the values found in the header file for you microcontroller. So you could write the following:

#pragma vector = SPI_TXE_vector

instead of:

#pragma vector = 0x0c

If you are using the same method for multiple vectors then you can provide the list of vector numbers as a comma separated list thus:

#pragma vector = SPI_TXE_vector, TIM1_OVR_UIF_vector

Something which is not obvious from the above code is the fact the an ISR cannot take any parameters nor can it return a value. Hence the method definition takes a void parameter list and returns void.

You should also note that it is possible to write an interrupt method (i.e. decorate a method with __interrupt) without providing a #pragma vector statement. In this case the compiler will not generate an entry in the interrupt vector table.

You should also consider how fast the ISR needs to be. In this case, a single button press, we do not need any real efficiency as the microcontroller is not really doing anything overly complex. This may not be the case in non-trivial applications. In which case you will need to analyse your system to determine how fast the ISR needs to be. You also need to take into account the fact that one ISR may be interrupted by a higher priority ISR.

Setting/Changing interrupt priorities

Interrupts can be assigned a software priority from 0 to 3. This allows the interrupts to be nested ensuring that the most important interrupts receive attention over less important interrupts.

Interrupt priority is given by the following:

Where the priority increases from the top of the table towards the bottom.

By default, all of the interrupts are assigned the same software priority, 3. This effectively means that the priority is disabled and all interrupts are treated as equal by the software. When the software priority is disabled or when two or more interrupts have the same priority then the interrupts are queued for processing

The software interrupt priority for the interrupt vectors (and hence your ISR) is stored in a group of registers. Each register (ITC_SPR1 through ITC_SPR8) holds the priority for four interrupt vectors. At reset all of these priorities are reset to 11 – software priority disabled. The register mapping is given by the following table:

Changing the priority is a simple matter of identifying the vector and changing the appropriate register (ITC_SPR1_VECT1SPR for vector 1, ITC_SPR1_VECT2SPR for vector 2 etc.).

Conclusion

Much of the information we have covered here is adequate for the average user intending to develop a module for the Netduino GO!. For more advanced topics such as what happens in low power modes, how interrupts are handled, nested interrupt handling etc. then you should refer to Chapter 6 in RM0016.

A few days ago I was discussing a post from this series in the Netduino chat room with someone who is following the posts and is keen to learn about the STM8S. It became apparent that there are a few things I am taking for granted; namely:

• Assuming you all have the same hardware set up as me
• You are familiar with the development environment I am using

For the hardware case this is certainly unlikely especially with the availability and low pricing of the STM8S Discovery boards. As for the software, well there are at least two environments available and a number of compilers and assemblers.

The objective of this post is to describe the environment I am using and how you can convert the hardware and software setup to work with the STM8S Discovery board. By converting the application to this board we will cover the principle steps which should be followed in order to convert the application to run on any of the STM8S family of microcontrollers. We will also cover some of the shortcut keys in the development environment in order to help the novice/infrequent user of the environment become a little more productive.

For the purposes of this exercise we will look at the Simple GPIO example as this application is small and simple.

Hardware

The first thing we will do is look at the hardware I am using and then compare this to the STM8S Discovery board.

My Hardware Configuration

I am using the STM8S103F3P3 in a TSSOP20 package. This has been mounted on a TSSOP20 to DIP board to allow me to insert the chip into a breadboard circuit. When mounting this on a breadboard you need the following components:

• 2 x 1uF capacitors (you can get away with only one for simple circuits)
• 1 x 100nF capacitor
• 3.3V regulated power supply
• ST-Link/V2 programmer
• Some wire to wire it all up.

To put this together, place one of the 1uF capacitors between V_SS and V_CAP and a 100 nF capacitor is placed between V_DD and V_SS. An additional (this is the optional capacitor) 1uF capacitor is also placed across the +3.3V and ground of the power supply.

The ST-Link/V2 should be connected to 3.3V and ground with the SWIM and NRST lines connected to the appropriate pins on the STM8S.

When you have all this put together you will have something like the following:

STM8S103 set up on Breadboard

Now compare this to the STM8S Discovery board:

STM8S Discovery Board

As you can see, all of the work setting up the hardware has been done for you :). You will also note that the chip used is the STM8S105C6T6. This chip is a different series to the one I am targeting. It is also a larger package giving the developer access to more ports etc. This board also has the ST-Link programmer built into the board. The only thing we need in order to use this board is a USB cable.

Development Environment

The development environment we are using is the IAR Kickstarter environment. This allows the creation of small applications (8 KBytes) for the STM8S. At the time of writing, the licence allowed the creation of fully functional applications with no commercial restrictions on the applications you create. The only requirement for the developer is that you register for a licence key.

We will not cover setting up the environment as this is a standard Windows installer.

If you have decided to use this environment then you might want to check out Custom IAR Templates for STM8S Projects. Arron Chapman has also put together some additional files and templates together. These are available in his posting Custome IAR STM8S Template.

Running the Application

We should now have the hardware and software set up so let’s take a look at the application we will be working with:

#include <iostm8S103f3.h>

int main( void )
{
    //
    //  Initialise Port D.
    //
    PD_ODR = 0;             //  All pins are turned off.
    PD_DDR_DDR5 = 1;        //  Port D, bit 5 is output.
    PD_CR1_C15 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C25 = 1;         //  Pin can run up to 10 MHz.
    //
    //  Now lets toggle to IO line.
    //
    while (1)
    {
        PD_ODR_ODR5 = 1;    // Turn Port D, Pin 5 on.
        PD_ODR_ODR5 = 0;    // Turn Port D, Pin 5 off.
    }
}

If you are using the STM8S103F3 chip then the application should compile and deploy OK. So now let’s consider what we need to do to make this run on the STM8S Discovery board.

The first thing to note is the include file we are using. This should match the chip you are using in your environment. You can find a full list of the include files supplied with the development environment in the inc directory of the installation. On my machine these can be found in C:Program Files (x86)IAR SystemsEmbedded Workbench 6.0 Kickstartstm8inc. If you browse through this directory you will find a file iostm8s105c6.h. This is the file you should be using for the STM8S Discovery board.

So make the change to the include file and we can now compile the application (press F7). The application should compile without any warnings or errors.

If you were to try to deploy the application now you would receive an error from the development environment. This is because the development environment and the programmer (ST-Link/V2) are targeting the wrong chip. To correct this we need to change the project options. Assuming you have your development environment open you should be looking at something like this:

Simple GPIO Project In IAR

Right click on the project name in the left hand panel (Simple GPIO – Debug) and select Options. Look for the Device text on the dialog which appears and click on the button to the right of the text box which contains the text STM8S103F3P3 and follow the popups which appear and select the STM8S105C6 chip.

Using IAR Options to Change the Target Device

At this point we should be ready to try the application out. So make sure that the STM8S Discovery board is connected to your PC and compile and deploy the application (Ctrl-D). At this point the application compiles and deploys after which the debugger should stop at the default breakpoint which has been set for you on the first line of code. Your display should look something like the following:

Breakpoint on the first line of code in the IAR environment

The green arrow and the highlighted line indicate the next line of code to be executed. From here we have a few options open to us:

• Run the application with no breakpoints
• Single step through the application
• Set some breakpoints and then run the application

These options should be familiar to any software engineer.

Let’s hook the STM8S Discovery board up to an oscilloscope and run the application (press F5). You should now see the following output on the oscilloscope:

Simple GPIO Running on the STM8S Discovery Board Oscilloscope Output

IAR Shortcut Keys

Throughout this post we have looked at some of the shortcut keys which I commonly use. The following table shows the key and the description of its function. This is not meant to be a comprehensive list of the keys used but it covers a few of the basics which I use regularly:

Conclusion

As we have shown, a few simple changes to the example code in The Way of the Register series are all we need to make to run these examples on other STM8S chips other than the STM8S103F3. In summary we need to do the following:

• Check the include file and make sure it matches the chip you are using
• Check the chip the development environment is targeting

Hope you have found this useful and continue to enjoy The Way of the Register series.

In a recent post we looked at the generation of a square wave signal using a timer and the update/overflow interrupt. There we generated a 20 Hz signal from the STM8S by toggling an output port using direct access to a port configured as an output port. In this post we will go one step further and use the capabilities of the timer to generate the pulse directly. We will also look at how we can manipulate the registers to allow the generation of a PWM pulse from the STM8S by simply changing the register values used to configure the port.

It is important that you read and understand the previous post Using Timers on the STM8S before continuing further as we will be using much of the knowledge in that post here.

So the project definition here is simple to start with, we will generate a square wave without using GPIO ports. We will then follow up on this by changing the values in the registers to generate a PWM signal with a duty cycle which can be defined by the programmer.

The Registers

The application will use most of the registers described in the previous post as well as the following:

1. TIM2_CCR1H & TIM2_CCR1L – Capture/Compare Register 1 High/Low
2. TIM2_CCER1 – Capture/Compare Enable register 1
3. TIM2_CCMR1 – Capture/Compare Mode Register 1

TIM2_CCR1H & TIM2_CCR1L – Capture/Compare Register 1 High/Low

These two registers are analogous to the TIM2_ARRH and TIM2_ARRL registers. TIM2_ARRH/L are used to determine the period of the signal whilst TIM2_CCR1H/L are used to determine the duty cycle of the signal. Let us assume that we are using the value of 50,000 for TIM2_ARRH/L as in the last post then by setting TIM2_CCR1H/L to 25,000 will give a duty cycle of 50%. Similarly, setting TIM2_CCR1H/L to 12,500 with give a duty cycle of 25% (or 75%) depending upon the register settings for active high/low – see TIM2_CCER1.

TIM2_CCER1 – Capture/Compare Enable register 1

We will be using two bits in this register, Capture/Compare 1 Output Polarity (CC1P) and Capture/Compare output Enable (CC1E).

So let’s start with the easy one, CC1E. This simply enables or disables the capture/compare for channel 1 of Timer 2. Setting this to 1 enables the mode, setting this to 0 disables the mode.

On to the difficult bit of this register, namely the output polarity (CC1P). This bit determines the polarity of the active state. A value of 1 configures the active state to be low whilst a value of 0 configures the state to be high.

It is important to note here that the meaning of active is different from the meaning of a high or low signal. Let us consider a simple example, namely a PWM signal with a duty cycle of 50%. So, for 50% of the time the signal is logic 1 (high) and for 50% of the time the signal is logic 0 (low). Or another way of looking at it is that if we define high to be active and low to be inactive then for 50% of the time the signal is active and 50% of the time the signal is inactive.

CC1P allows us to define what we mean by active and inactive. Once we have the application written we can change this value and see the effect on the output.

TIM2_CCMR1 – Capture/Compare Mode Register 1

This register allows the application to change the way in which the channel is configured. In this case we will only be concerned with setting this to one of two values, namely 6 or 7.

Software

The first two things we will do is steal some code from previous posts, namely the Configuring the System Clock and Using Timers on the STM8S. We will use the InitialiseSystemClock and InitialiseTimer2 methods respectively.

The next thing we need to consider is how we set up the timer. We will continue to use Timer 2 so we can again use some of the code from previous posts. However, we need to make a few modifications to SetupTimer2 method.

So let’s start by having a 25% duty cycle (25% high, 75% low). At the moment we are not too worried about the frequency of the signal so let’s work with TIM2_ARRH/L set to 50,000 as in the previous post. This means that we want the output low for 75% of the time (37,500 counts) and high for 25% of the time (12,500 counts). Time for the first decision, let’s use PWM mode 1 (TIM2_CCMR1_OC1M = 6).

Given the default mode (down-counting) and looking at the register definition for TIM2_CCMR1_OC1M we want to define active as a logic 0 and inactive as a logic 1. So this means we need to set TIM_CCMR1_OC1M = 0.

If we put all of this together we end up with the following method:

void SetupTimer2()
{
    TIM2_PSCR = 0x00;       //  Prescaler = 1.
    TIM2_ARRH = 0xc3;       //  High byte of 50,000.
    TIM2_ARRL = 0x50;       //  Low byte of 50,000.
    TIM2_CCR1H = 0x30;      //  High byte of 12,500
    TIM2_CCR1L = 0xd4;      //  Low byte of 12,500
    TIM2_CCER1_CC1P = 0;    //  Active high.
    TIM2_CCER1_CC1E = 1;    //  Enable compare mode for channel 1
    TIM2_CCMR1_OC1M = 6;    //  PWM Mode 1 - active if counter < CCR1, inactive otherwise.
    TIM2_CR1_CEN = 1;       //  Finally enable the timer.
}

And as we are using a timer to do all of the work, our main method becomes:

void main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    InitialiseTimer2();
    SetupTimer2();
    __enable_interrupt();
    while (1)
    {
        __wait_for_interrupt();
    }
}

Let’s Look at the Output

If we look at the clock settings (16MHz) and the value for TIM2_ARR (50,000) we should be looking for a signal with a frequency of around 320 Hz (16 MHz / 50,000). So compiling the above we are expecting a 320Hz signal with a duty cycle of 25%. Here is the output on the oscilloscope:

PWM with a 25% high signal

Changing the value of TIM2_CCER1_CC1P to 1 gives the following:

PWM with a 75% high signal

As you can see, the polarity allows us to change what we mean by active.

Some ideas for you:

1. Change TIM2_ARR values to change the frequency
2. Change TIM2_CCR1 values to change the duty cycle

As always, the source code for this example is available for download.

Source Code Compatibility

System	Compatible?
STM8S103F3 (Breadboard)
Variable Lab Protomodule
STM8S Discovery