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Posts Tagged ‘The Way of the Register’

STM8S SPI Slave (Part 2)

Monday, November 19th, 2012

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 Logic Analyser

      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

      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

The Way of the Register Source Code Update

Sunday, October 7th, 2012

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:

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

Single Conversion ADC on the STM8S

Monday, September 17th, 2012

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:

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 &amp; 0xff;
    high = (reading >> 8) &amp; 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

Converting The Way of The Register Examples

Friday, August 31st, 2012

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 VSS and VCAP and a 100 nF capacitor is placed between VDD and VSS. 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

STM8S103 set up on Breadboard

Now compare this to the STM8S Discovery board:

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

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

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

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

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:

Key Function
Ctrl-D Compile the code and deploy to the device.
Shift-Ctrl-D Terminate the debugging session.
F7 Make the project, do not deploy.
F5 In debug mode, causes the application to run until the next breakpoint is reached or the application terminates.
F10 In debug mode, execute the currently selected statement and then break at the next line of code. This will step over any method calls.
F11 As with F10 but this time step into any method calls.
Shift-F11 Step out of the current method being debugged and return to the calling method with a breakpoint set accordingly.
Shift-Ctrl-R Reset the debugging environment setting a breakpoint on the first line of code and restart the application.
Ctrl-K Comment out the currently selected lines of code.
Shift-Ctrl-K Uncomment the selected lines of code.

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.

Generating a Regular Pulse Using Timer 2

Wednesday, August 29th, 2012

In previous posts you may have seen an example program where we generate a 20Hz signal using the overflow interrupt on Timer 2. Here we will translate the post to use direct register access rather than use the STD Peripheral Library.

So the project definition is simple, output a regular signal (20Hz with a 50% duty cycle) on Port D, Pin 4 (i.e. pin 2 on the STM8S103F3P3).

Algorithm

To make this project a low on processor power we will use interrupts to generate the pulse. To do this we will make use of one of the STM8S timers, namely Timer 2 (T2). The algorithm becomes:

  1. Turn off the timer
  2. Setup the timer to generate an interrupt every 1 / 40th of a second
  3. Set up the output port to generate the signal.
  4. Wait for interrupts indefinitely

The Interrupt Service Routine (ISR) then has one very simple task, toggle the output port and wait for the next interrupt.

The Registers

This application is simple and really only uses a fraction of the power of the STM8S timers. In fact we can set up the chip using relatively few registers. With the exception of resetting the timer to a known state we will be using only six registers in this exercise:

  1. TIM2_PSCR
  2. TIM2_ARRH and TIM2_ARRL
  3. TIM2_IER
  4. TIM2_CR1
  5. TIM2_SR1

TIM2_PSCR – Timer 2 Prescalar

The 16-bit counter in Timer 2 receives a clock signal from the prescalar. This in turn receives a clock from the internal clock of the STM8S (i.e. fmaster). The prescalar divides the fmaster clock by the prescalar set in the TIM2_PSCR register. This allows the timer to receive a slower clock signal than that running the STM8S. The prescalar is a power of 2 and the effective frequency of the clock running Timer 2 is given by the following formula:

fcounter = fmaster / 2TIM2_PSCR

where fcounter is the frequency of the signal being used as a clock source for Timer 2.

TIM2_PSCR is a 4 bit number and this restricts the value of the prescalar to 1 to 32,768.

We will come back to this formula when we write the software in order to calculate the prescalar we will need to generate the 20Hz clock signal.

TIM2_ARRH and TIM2_ARRL – Counter Auto-Reload Registers

We will be using the counter as a simple up/down counter. We will be loading this register with a counter value which the timer will count up to / down from. An interrupt will be generated when the counter value has been reached (for up) or zero is reached (for down). The counter will then be reset using the values in these two registers.

The only important thing to note about these two registers is that TIM2_ARRH must be loaded with a value before TIM2_ARRL.

TIM2_IER – Interrupt Enable Register

This register determines which interrupts Timer2 can generate. In our case we only need one, namely the update interrupt. This is generated when the counter value has been reached.

The interrupt is enabled by setting TIM2_IER_UIE to 1.

TIM2_CR1 – Timer 2 Control Register 1

The only bit we will be interested here is the Counter ENable bit (CEN). This will be used to start the counter.

TIM2_SR1 – Timer 2 Status Register 1

This register gives us status information about the timer. There is only one bit we are interested in for this exercise and that is the Update Interrupt Flag (UIF). This bit determines if an update interrupt is pending. The bit is set by hardware but crucially it must be reset by software.

When we enter the ISR, this bit will have been set by the hardware controlling the timer. On existing the ISR the hardware will check the status of the bit. If it is set then the interrupt will be generated once more. This means that if we are not careful then we can end up in a cycle of generating an interrupt, processing the interrupt in the ISR and then generating the interrupt again ad infinitum. It is therefore crucial that this bit is cleared before the ISR is exited.

Software

One of the first things to note is that as with all of the examples we will discuss in this series, we will assume a clock running using the internal oscillator and set to 16MHz.

The code which will deal with the interrupt has a very simple job to do, namely toggle the pin we are using to generate the output pulse. One thing to note it that as we are toggling the pin in this method we will effectively be halving the output frequency of the signal which has been generated. Lets look at what is happening.

  1. ISR 1 – output is low we will make the output high.
  2. ISR 2 – output is high we will make the signal low
  3. ISR 3 – output is low we will make the signal high
  4. etc.

The frequency of the output for a regular signal is determined by the amount of time between the two rising edges of the output. So in our case, the time is double the frequency of the calls to the ISR as we toggle the output in the ISR. This is important and will be used in the calculations we make regarding the timer settings later.

The remainder of the code looks similar to that used in the external interrupts example presented in an earlier post.

//
//  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.
}

If you have been following the series, the next piece of code should also be familiar (see the Simple GPIO example). We will be setting up Port D, pin 4 to be an output port. This is the pin which will output the signal we will be generating.

//
//  Setup the port used to signal to the outside world that a timer even has
//  been generated.
//
void SetupOutputPorts()
{
    PD_ODR = 0;             //  All pins are turned off.
    PD_DDR_DDR4 = 1;        //  Port D, pin 4 is used as a signal.
    PD_CR1_C14 = 1;         //  Port D, pin 4 is Push-Pull
    PD_CR2_C24 = 1;         //  Port D, Pin 4 is generating a pulse under 2 MHz.
}

The next method resets Timer 2 and put it into a known state. This simply requires resetting all of the Timer 2 registers to 0.

//
//  Reset Timer 2 to a known state.
//
void InitialiseTimer2()
{
    TIM2_CR1 = 0;               // Turn everything TIM2 related off.
    TIM2_IER = 0;
    TIM2_SR2 = 0;
    TIM2_CCER1 = 0;
    TIM2_CCER2 = 0;
    TIM2_CCER1 = 0;
    TIM2_CCER2 = 0;
    TIM2_CCMR1 = 0;
    TIM2_CCMR2 = 0;
    TIM2_CCMR3 = 0;
    TIM2_CNTRH = 0;
    TIM2_CNTRL = 0;
    TIM2_PSCR = 0;
    TIM2_ARRH  = 0;
    TIM2_ARRL  = 0;
    TIM2_CCR1H = 0;
    TIM2_CCR1L = 0;
    TIM2_CCR2H = 0;
    TIM2_CCR2L = 0;
    TIM2_CCR3H = 0;
    TIM2_CCR3L = 0;
    TIM2_SR1 = 0;
}

The next thing we need is a method which sets the Timer 2 to generate the interrupt. This is where we need to start doing some calculations.

So let’s start with the frequency of the clock going into the counter for Timer 2. As we have seen earlier, this is given by the following:

fcounter = fmaster / 2TIM2_PSCR

Now we also know that the interrupts will be generated every time the counter value is reached. So the frequency of the interrupt is given by the following:

finterrupt = fcounter / counter

Putting the two together we get the following:

finterrupt = fmaster / (2TIM2_PSCR * counter)

A little rearranging gives:

(2TIM2_PSCR * counter) = fmaster / finterrupt

If we plug in the numbers we know, fmaster = 16MHz and finterrupt = 40 (remember that the frequency of the signal we are generating is half the frequency of the interrupts) then we find:

(2TIM2_PSCR * counter) = 400,000

So, if we take 400,000 and divide by 50,000 (for simplicity) then we have a factor of 8. So, given that the counter is a 16-bit counter then the counter should be 50,000 and the prescalar should be 3 (23 = 8).

//
//  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.
}

Now we have all of the component parts we simply need to call the methods to set everything up and then wait for the interrupts to fire. So our main method looks like this:

//
//  Main program loop.
//
int main( void )
{
    __disable_interrupt();
    SetupOutputPorts();
    InitialiseTimer2();
    SetupTimer2();
    __enable_interrupt();
    while (1)
    {
        __wait_for_interrupt();
    }
}

Running this application results in the following trace on the oscilloscope:

20 Hz Square Wave Output on an Oscilloscope

20 Hz Square Wave Output on an Oscilloscope

A quick look at the measurements shows that this application is generating a 20Hz signal. If you don’t have a scope then you can hook a LED (use the LED circuit from the previous article on external interrupts) through a transistor. You should be able to see the LED flicker as it is turned on and off. You could also slow down the rate of the pulses by looking at changing the prescalar or adding a counter.

As always, the source code is available for download.

Source Code Compatibility

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

Using the UART on the STM8S

Monday, August 27th, 2012

We have previously seen how to configure the STM8S using the STD Peripheral Library using both the high level API and the slightly lower level register access. In this post we will use the definitions in the iostm8s103f3.h and we will also have a look at the registers which control the UART ending with a program which uses the UART to send data to a terminal emulator on a desktop computer (in my case a PC). We must remember that while this article looks at low speed communications with a PC, the UARTs on the STM8S have a variety of uses covering other protocols other than those discussed in this article.

The definition of the problem is simple, allow the STM8S to send debug information to a terminal emulator running at 115200,n,8,1 (for all of those who remember DOS MODE commands for serial communication).

It should be noted that UART1 is not available on the STM8S Discovery board and so UART2 is used instead.

The Registers

In order to set up the UART we will need to perform the following tasks:

  1. set the parity and number of data bits
  2. set the parity
  3. set the number of stop bits
  4. setup the baud rate
  5. set the clock polarity etc.

It is important to remember that transmission and reception must both be disabled before we start to change these registers.

UART_CR1 – Data Bits and Parity

The number of data bits is selected using the M bit of CR1. This can be set to either 8 or 9 bits. We will be using 8 data bits and so will need to set UART_CR1_M to 0 (setting to 1 would enable 9 data bits).

To set the parity we would set PCEN and PS. In our case we are disabling parity so only need to worry about PCEN (Parity Control Enable). Setting this bit to 0 will disable the parity calculation.

UART_CR3 – Number of Stop Bits and Clock Settings

The number of stop bits can be set to 1, 1.5 or 2. This is controlled by setting UART_CR3_STOP to one of the following values:

Value Description
00 1 Stop bit
01 Reserved
10 2 Stop bits
11 1.5 Stop bits

Next, we need to consider the clock settings.

UART_CR3_CPOL determines the idle state of the clock, 0 sets the clock low when idle, 1 sets it high when idle.

UART_CR3_CPHA determines if the data should be stable on the rising or falling edge of the signal. Setting this to 0 means the data is set on the rising edge of the clock signal. Setting this to 1 means the data is ready on the falling edge of the clock signal.

UART_CR3_LBCL determines if the clock pulse for the last data bit is set to the clock pin. A 0 means the last clock pulse is not generated while 1 means that the pulse is generated.

UART_BRR1 & UART_BRR2 – Baud Rate Registers

The baud rate of the UART is controlled by dividing fmaster by the baud rate divisor. The result gives the clock speed of the serial port. In our case we have a 16 MHz clock speed for the microcontroller and we want the serial port to run at 115200 baud. So some simple rearranging of the formula and the UART divider is given by:

UART Divider = fmaster / baud rate
= 16,000,000 / 115,200
= 138
= 0x008a

Now we need to rearrange the number 0x008a a little in order to get the right bits into BRR1 and BRR2 (Baud Rate Register 1 & 2). This was written as a 32 bit number to illustrate how this is put into the registers. To do this we split the number (represented by d3d2d1d0) into three parts:

  • the first digit (d3) – 0
  • the next two digits (d2d1) – 08
  • the last digit (d0) – a

And set up the registers as follows:

BRR1 = d2d1
= 0x08
BRR2 = d3d0
= 0x0a

When setting these registers it is important to remember to set BRR2 before setting BRR2.

UART_CR2 & UART_CR3 – Enabling the UART

The first action we would need to take is to disable the UART and the last thing we should do is enable it. This is controlled by three bits in two different registers, namely registers UART_CR3 and UART_CR2. All three bits use 0 for disable and 1 for enable. The bits we been to set are:

UART_CR2_TEN Enable/disable transmission
UART_CR2_REN Enable/disable reception
UART_CR3_CKEN Enable/disable the clock

In our case we do not need to enable the output of the system clock or reception. The software below will enable these anyway in order to provide a generic serial initialisation method which can be using a variety of circumstances.

Software

Moving on to our software, we will need a standard STM8 project for the microcontroller you are using. I am using the STM8S103F3P3 and the default project I set up in a previous article.

Setting Up the UART

This code makes a fundamental assumption, namely that you have configured the chip and your circuit to run at 16 MHz. I did this by setting the chip to use the internal oscillator as its clock source and using a prescalar of 1 (see the previous article on setting up the system clock for more information). The code for this is in the InitialiseSystemClock method.

The next step is to configure the UART. This is performed in the InitialiseUART method.

//
//  Setup the UART to run at 115200 baud, no parity, one stop bit, 8 data bits.
//
//  Important: This relies upon the system clock being set to run at 16 MHz.
//
void InitialiseUART()
{
    //
    //  Clear the Idle Line Detected bit in the status register by a read
    //  to the UART1_SR register followed by a Read to the UART1_DR register.
    //
    unsigned char tmp = UART1_SR;
    tmp = UART1_DR;
    //
    //  Reset the UART registers to the reset values.
    //
    UART1_CR1 = 0;
    UART1_CR2 = 0;
    UART1_CR4 = 0;
    UART1_CR3 = 0;
    UART1_CR5 = 0;
    UART1_GTR = 0;
    UART1_PSCR = 0;
    //
    //  Now setup the port to 115200,n,8,1.
    //
    UART1_CR1_M = 0;        //  8 Data bits.
    UART1_CR1_PCEN = 0;     //  Disable parity.
    UART1_CR3_STOP = 0;     //  1 stop bit.
    UART1_BRR2 = 0x0a;      //  Set the baud rate registers to 115200 baud
    UART1_BRR1 = 0x08;      //  based upon a 16 MHz system clock.
    //
    //  Disable the transmitter and receiver.
    //
    UART1_CR2_TEN = 0;      //  Disable transmit.
    UART1_CR2_REN = 0;      //  Disable receive.
    //
    //  Set the clock polarity, lock phase and last bit clock pulse.
    //
    UART1_CR3_CPOL = 1;
    UART1_CR3_CPHA = 1;
    UART1_CR3_LBCL = 1;
    //
    //  Turn on the UART transmit, receive and the UART clock.
    //
    UART1_CR2_TEN = 1;
    UART1_CR2_REN = 1;
    UART1_CR3_CKEN = 1;
}

We will need to provide a method of sending a simple string to the serial port. The algorithm is simple:

  1. Set a pointer to the start of the string
  2. If the character pointed to be the pointer is not a null (i.e. 0) character then
    1. Transfer the character pointed to be the pointer into the UART data register
    2. Wait until the data register has been sent (Transmission Empty is true)
    3. Move the pointer on one byte

//
//  Send a message to the debug port (UART1).
//
void UARTPrintf(char *message)
{
    char *ch = message;
    while (*ch)
    {
        UART1_DR = (unsigned char) *ch;     //  Put the next character into the data transmission register.
        while (UART1_SR_TXE == 0);          //  Wait for transmission to complete.
        ch++;                               //  Grab the next character.
    }
}

And finally we need a main program to control the application.

//
//	Main program loop.
//
void main()
{
	__disable_interrupts();
	InitialiseSystemClock()
    InitialiseUART()
	__enable_interrupts();
	while (1)
	{
        UARTPrintF("Hello from my microcontroller....\n\r");
		for (long counter = 0; counter < 250000; counter++);
	}
}

The full application code can be downloaded from here. Simply unzip the files and open the project with IAR. The application can be downloaded to the chip by pressing Ctrl-D. Once downloaded to the microcontroller press F5 to run the application.

This application has been tested on my reference platform, the Variable Labs Protomodule and the STM8S Discovery board.

To check the application is working, connect the Tx line of the STM8S (in my case pin 2) to the Rx line on a PC which can accept 3.3V TTL logic signals. This is important, the port must accept 3.3V TTL and NOT standard RS232 signals. To do this I use a 3.3V FTDI cable and connect this to the STM8S and one of the USB ports on my PC. This gives me a COM port which I can connect to devices running at 3.3V. Now open up a terminal emulator (I used PuTTY) and connect to the com port using the correct protocol. You should see the following output:

Output from the STM8S shown on a PuTTY terminal

Output from the STM8S shown on a PuTTY terminal

Conclusion

Adding these methods to you project should allow you to generate debug output from an application running on the STM8S or communicate with devices which are controlled using a serial communication protocol.

Source Code Compatibility

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

External Interrupts on the STM8S

Thursday, August 16th, 2012

In a previous post we looked at the GPIO pins and how we could set the pins to either input or output but we concentrated on output. In this post we will look at using interrupts to detect input from the user and an output to indicate that the input has been detected. Our objective is:

  1. Create a circuit which can register when a button has been pressed.
  2. Toggle a LED when the button is pressed

Should be simple, so let’s give it a go.

Hardware

For the hardware we will need three components:

  1. STM8S Circuit
  2. Switch
  3. LED

STM8S Circuit

For the STM8S we need the STM8S103F3 connected to 3.3V and ground. We will also need to add two ceramic capacitors, a 100nF between Vss and Vdd (pins 7 and 9) and a 1uF between Vss and Vcap (pins 7 and 8).

Switch Circuit

For the switch circuit we need a switch and a pull-up resistor. We will pull the input line to the STM8S high through the pull-up resistor. Pressing the switch will pull the input line low. This part of the circuit looks like this:

This circuit will have the pin held high until the switch is pressed. All we need to do is to detect the falling edge of the input to the STM8S and we have a way to detecting when the user presses the switch.

One thing to note here is that we will be ignoring switch bounce and so we are likely to get the odd spurious message. We will come back to this later.

LED Circuit

This circuit is a simple transistor circuit which is used to switch on a LED. By applying a low current to the base of a transistor we can switch on a larger current to drive the LED. A quick simple circuit looks like this:

By setting a GPIO pin connected to the base of the transistor to high we can turn on a LED.

Putting it all together, we connect the switch to PD4 and the LED to PD3.

Software

The software is a simple extension of the previous post on GPIO pins. Here we are going to use the same port (port D) for both input and output. The full code is:

#include <intrinsics.h>
#include <iostm8s103f3.h>

//
//  Process the interrupt generated by the pressing of the button on PD4.
//
#pragma vector = 8
__interrupt void EXTI_PORTD_IRQHandler(void)
{
    PD_ODR_ODR3 = !PD_ODR_ODR3;     //  Toggle Port D, pin 3.
}

//
//  Main program loop.
//
void main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    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 up to 10 MHz.
    //
    //  Now configure the input pin.
    //
    PD_DDR_DDR4 = 0;        //  PD4 is input.
    PD_CR1_C14 = 0;         //  PD4 is floating input.
    //
    //  Set up the interrupt.
    //
    EXTI_CR1_PDIS = 2;      //  Interrupt on falling edge.
    EXTI_CR2_TLIS = 0;      //  Falling edge only.
    __enable_interrupt();

    while (1)
    {
        __wait_for_interrupt();
    }
}

So let’s start and break the program down. The first thing you will notice is that we initially configure all of the pins on port D as outputs as we have previously seen:

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 up to 10 MHz.

The next thing we do is to set up the system to allow PD4 as an input pin:

PD_DDR_DDR4 = 0;        //  PD4 is input.
PD_CR1_C14 = 0;         //  PD4 is floating input.

The final step of the configuration is to set the external interrupt for the port to detect the falling edge of the signal:

EXTI_CR1_PDIS = 2;      //  Interrupt on falling edge.
EXTI_CR2_TLIS = 0;      //  Falling edge only.

The final part of the main program is to wait for an interrupt. This is repeated infinitely:

while (1)
{
    __wait_for_interrupt();
}

The interesting part of the problem is the interrupt routine as this is where the real work is actually done. The code is simple enough you just have to make sure that you declare the method correctly:

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

The first two lines declare the method as an Interrupt Service Routine (ISR). The #pragma vector = 8 tells the compiler which interrupt this method will be servicing. In this case, this method will be called to process the interrupts for Port D. We will look into ISRs a little more in a later post.

Putting it all Together

If we wire all of the hardware together on a breadboard we end up with something like the following:

and a quick video of it working:

Switch Bounce

As noted earlier, mechanical switches are susceptible to switch bounce. This is caused by the mechanical contacts not closing perfectly when the switch is pressed. You can notice an example of bounce in the above video. When the switch is pressed the second time you can see that the LED is switched off but it is then switched on straight away. This is the effect of the switch contacts causing multiple signals. If a scope is hooked up to a switch you see something like the following:

In this case the switch is held down and then released. Notice the spike; this is caused by the switch bounce. If the signal is large enough then the microcontroller will think that the switch has been pressed twice rather than once – in fact that is what happened in the above video. There are a few ways to solve this but I’ll leave that as an exercise to the reader.

You can find the full source for the above project here. This application has been tested on tmy reference platform, the Variable Labs Protomodule and the STM8S Discovery board.

Compatibility

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

Simple GPIO – The Way of the Register

Sunday, July 8th, 2012

If you have been following this blog for a while you will be aware that I have recently changed the way I write code for the STM8S from using the STD Peripheral Library to using direct register access. This has required that I go back to basics learning a new way to perform tasks I had only just mastered. The Way of the Register posts will show you have to control the STM8 using register access.

This first post will look at a simple task, controlling GPIO lines. The aim of this first exercise is to simply toggle one of the GPIO lines.

Hardware

For this task we need very little hardware, only the STM8S set up on breadboard. For this we need a minimum of two capacitors and the chip itself. A 1uF capacitor is placed between VSS and VCAP and a 100 nF capacitor is placed between VDD and VSS. An additional 1uF capacitor is also placed across the +3.3V and ground of the power supply. The following shows this set up on breadboard along with the breadboard power supply, connections to the oscilloscope and the ST-Link programmer:

STM8S103 set up on Breadboard

STM8S103 set up on Breadboard

The Registers

Before we can start coding we need to have a look at the registers we will be using. The GPIO pins have five registers used to control the GPIO function and set/read the value of a pin. These are:

Register
Data Direction Register
Control Register 1
Control Register 2
Output Data Register
Input Data Register

The following sections provide a brief description of the registers. For more information you should consult RM0016 – STM8S Reference Manual.

Px_DDR – Data Direction Register

This register determines which or the GPIO pins are outputs and which are inputs. A value of one in a bit sets a port line to output and zero sets the port line to input. You can set the value of the whole register or each individual bit in the register.

To set the whole register in one go you would execute something like the following:

PD_DDR = 0x0f;

This sets bits 0-3 inclusive of port D to output whilst bit 4-7 are inputs.

An alternative to this is to set up each GPIO pin individually, so the equivalent of the above is:

PD_ODR = 0;
PD_DDR_DDR0 = 1;
PD_DDR_DDR1 = 1;
PD_DDR_DDR2 = 1;
PD_DDR_DDR3 = 1;

Px_CR1 & PxCR2 – Control Register 1 and 2

These two registers determine the properties of the pins depending upon the operating mode of the pin. The following table summarises the operating modes:

Register Mode Value Description
CR1 Input 0 Floating input
CR1 Input 1 Input with pull-up
CR1 Output 0 Open drain
CR1 Output 1 Push-Pull
CR2 Input 0 Interrupt disabled
CR2 Input 1 Interrupt enabled
CR2 Output 0 Output up to 2 MHz.
CR2 Output 1 Output up to 10 MHz

In our case, we need the output pin to operate in Push-Pull mode and as we do not know the final speed of the output signal we will set this to the maximum, 10 MHz.

As with the data direction register, each bit in the registers maps to a specific GPIO on each port. In the case of these registers the bits are labelled C10-C17 (for Control Register 1) and C20-C27 (for control Register 2). The format appears to be CRB, where C stands for Control Register; R is an integer representing the register number and B is an integer representing the bit to be set.

In our case, this results in the following code:

PD_CR1_C14 = 1;
PD_CR2_C24 = 1;

Px_ODR – Input/Output Data Registers

These two registers allow you to read the value of an input pin and set the value of an output pin. As with the previous registers you can set/read the register in its entirety or bit by bit. So to turn all of the outputs off you would execute something like:

PD_ODR = 0;


In the case we have been using we will be outputting data on pin 4 of port D. So to set the pin high you would execute the following statement:

PD_ODR_ODR4 = 1;

Software

Now we have the theory in place lets put together a simple application which uses it. The following code sets up port D to have pin 4 configured as an output operating to 10 MHz. The main program loop then simply keeps flipping the bit. The result should be a square wave signal.

#include <iostm8S103f3.h>

int main( void )
{
    //
    //  Initialise Port D.
    //
    PD_ODR = 0;             //  All pins are turned off.
    PD_DDR_DDR4 = 1;        //  Port D, bit 4 is output.
    PD_CR1_C14 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C24 = 1;         //  Pin can run upto 10 MHz.
    //
    //  Now lets toggle to IO line.
    //
    while (1)
    {
        PD_ODR_ODR4 = 1;    // Turn Port D, Pin 4 on.
        PD_ODR_ODR4 = 0;    // Turn Port D, Pin 4 off.
    }
}

If we hook an oscilloscope up to PD4 on the STM8 chip we see the following:

Simple GPIO Output

Simple GPIO Output

As you can see, we have a square wave. Note that the wave is not symmetrical. This is due to the branch instruction being executed in the while loop to go back to the start of the loop.

Source Code

Here is the IAR project and source code for this post. This code has been tested on my reference platform, Variable Lab Protomodule and the STM8S Discovery board.

Compatibility

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

Using Registers on the STM8S

Saturday, June 23rd, 2012

A few weeks ago I had a rant about the STM8S Standard Peripheral library after it cost me a fair amount of time tracking down what appeared to be a bug in the library. As a result of this I have moved over to accessing the registers on the chip directly to control the operation of the chip. A recent question ion one of the forums I haunt has prompted this post. Here I am going to present a few different ways of achieving the same task, one using the STD Peripheral Library and two examples using the registers directly but in different ways.

The task we are going to be looking at is one I perform as part of my initialisation of the STM8S, namely I set the clock up to a known state. In this case we will be setting the system to use the HSI clock running at 16 MHz with no dividers.

It is important to note that you will need to have a copy of the reference manual for the chip available when using direct register access to control the microcontroller. In this case you should be looking for document RM0016 on ST’s web site.

Using the Standard Peripheral Library

Using the STD Peripheral Library makes this a relatively simple task. We only need to call four methods:

CLK_DeInit();
CLK_SYSCLKConfig(CLK_PRESCALAR_CPUDIV1);    // CPU Prescalar = 1.
CLK_SYSCLKConfig(CLK_PRESCALAR_HSIDIV1);    // Prescalar = 1, 16 MHz.
CLK_ClockSwitchConfig(CLK_SWITCHMODE_AUTO,  // Automatically switch
                      CLK_SOURCE_HSI,       // Switch to internal timer.
                      DISABLE,              // Disable the clock switch interrupt.
                      CLK_CURRENTCLOCKSTATE_DISABLE);   // Disable the previous clock.

The headers for the above methods can be found in the file stm8s_clk.h and the source code for the methods can be found in the file stm8s_clk.c. These source files can be found in the source code folder of the STD Peripheral Library.

Direct Register Access – Method 1

This first method of accessing the registers continues to use the STD Peripheral Library files but we do not make any calls into the library. Instead we use the definitions for the registers to access the chip directly.

So let’s start by breaking the above four methods down into their direct register equivalents.

CLK_DeInit

The first thing we need to do is to reset all of the registers to the default values:

CLK->ICKR = CLK_ICKR_RESET_VALUE;
CLK->ECKR = CLK_ECKR_RESET_VALUE;
CLK->SWR  = CLK_SWR_RESET_VALUE;
CLK->SWCR = CLK_SWCR_RESET_VALUE;
CLK->CKDIVR = CLK_CKDIVR_RESET_VALUE;
CLK->PCKENR1 = CLK_PCKENR1_RESET_VALUE;
CLK->PCKENR2 = CLK_PCKENR2_RESET_VALUE;
CLK->CSSR = CLK_CSSR_RESET_VALUE;
//
//  The following set has to be performed twice.
//
CLK->CCOR = CLK_CCOR_RESET_VALUE;
while ((CLK->CCOR & CLK_CCOR_CCOEN) != 0);
CLK->CCOR = CLK_CCOR_RESET_VALUE;
CLK->HSITRIMR = CLK_HSITRIMR_RESET_VALUE;
CLK->SWIMCCR = CLK_SWIMCCR_RESET_VALUE;

As you can see, the library certainly hides a large amount of work from you. Most of the above code is simply a case of setting up the registers to default values.

To find out how this all works you need to start looking in the stm8s.h file. A quick search for CLK soon leads you to the following type declaration:

typedef struct CLK_struct
{
  __IO uint8_t ICKR;     /*!> Internal Clocks Control Register */
  __IO uint8_t ECKR;     /*!> External Clocks Control Register */
  uint8_t RESERVED;      /*!> Reserved byte */
  __IO uint8_t CMSR;     /*!> Clock Master Status Register */
  __IO uint8_t SWR;      /*!> Clock Master Switch Register */
  __IO uint8_t SWCR;     /*!> Switch Control Register */
  __IO uint8_t CKDIVR;   /*!> Clock Divider Register */
  __IO uint8_t PCKENR1;  /*!> Peripheral Clock Gating Register 1 */
  __IO uint8_t CSSR;     /*!> Clock Security System Register */
  __IO uint8_t CCOR;     /*!> Configurable Clock Output Register */
  __IO uint8_t PCKENR2;  /*!> Peripheral Clock Gating Register 2 */
  uint8_t RESERVED1;     /*!> Reserved byte */
  __IO uint8_t HSITRIMR; /*!> HSI Calibration Trimmer Register */
  __IO uint8_t SWIMCCR;  /*!> SWIM clock control register */
}
CLK_TypeDef;

If you have a look at the technical reference sheet for the chip, you will find section which shows the memory layout for the registers. These are at fixed locations in memory and should map to the above layout. The __IO is defined as volatile and will prevent the compiler from optimising out any references to the variables.

The next thing to note is that we still do not have a definition for CLK. A little more searching in the same file will lead you to the following statement:

#define CLK ((CLK_TypeDef *) CLK_BaseAddress)

So this defines CLK for us as a pointer to a location in memory. Some more searching (again in the same file) leads us to the following line of code:

#define CLK_BaseAddress         0x50C0

So the code CLK->ICKR = 0; will set the register at location 0x50C0 to zero.

One point to note is the statement while ((CLK->CCOR & CLK_CCOR_CCOEN) != 0);. This illustrates the use of another type of declaration you will find in stm8s.h, namely, CLK_CCOR_CCOEN. This declaration allows you to mask off certain bits within a register and use this to set or check values in a register. The name is made up of three parts:

Name Description
CLK Clock registers are being access.
CCOR This relates to the CCOR register.
CCOEN Mask the CCOEN bits in the register.

CLK_SYSCLKConfig

The next task is to set the prescalar for the system clock. This is being set to 1 to ensure the system runs at 16 MHz.

CLK->CKDIVR  &= (uint8_t) (~CLK_CKDIVR_HSIDIV);
CLK->CKDIVR |= (uint8_t) ((uint8_t) CLK_PRESCALAR_HSIDIV1 & (uint8_t) CLK_CKDIVR_HSIDIV);

The first line resets the prescalar to a known value whilst the second selects the divider which will be used.

CLK_ClockSwitchConfig

The final operation is to switch the system clock to the HSI and this is achieved with the following code:

CLK->SWCR |= CLK_SWCR_SWEN;
CLK->SWCR &= (uint8_t) (~CLK_SWCR_SWIEN);
CLK->SWR = (uint8_t) CLK_SOURCE_HSI;
uint16_t downCounter = CLK_TIMEOUT;
while ((((CLK->SWCR & CLK_SWCR_SWBSY) != 0 ) && (downCounter != 0)))
{
    downCounter--;
}

Direct Register Access – Method 2

This method uses the register declarations found in the header files provided by IAR. So for the STM8S103F3 we will be looking in the file .

CLK_DeInit

As before, the first thing we will do is to reset the registers to a known set of values:

CLK_ICKR = 0;
CLK_ECKR = 0;
CLK_SWR = 0xe1;
CLK_SWCR = 0;
CLK_CKDIVR = 0x10;
CLK_PCKENR1 = CLK_PCKENR1_SPI | CLK_PCKENR1_TIM2;   //  Enable the peripheral clocks we need.
CLK_PCKENR2 = 0;
CLK_CSSR = 0;
CLK_CCOR = 0;
while (CLK_CCOR_CCOEN != 0);
CLK_CCOR = 0;
CLK_HSITRIMR = 0;
CLK_SWIMCCR = 0;

The first thing you will notice is that by using this method we are not using the pointer dereferencing operator. Instead the application is accessing the registers directly. So let’s have a look at the header file and dissect the reset of the ICKR register. Searching for CLK_ICKR leads us to the following code:

typedef struct
{
  unsigned char HSIEN       : 1;
  unsigned char HSIRDY      : 1;
  unsigned char FHW         : 1;
  unsigned char LSIEN       : 1;
  unsigned char LSIRDY      : 1;
  unsigned char REGAH       : 1;
} __BITS_CLK_ICKR;
__IO_REG8_BIT(CLK_ICKR,    0x50C0, __READ_WRITE, __BITS_CLK_ICKR);

The first things we see is the definition of the structure which maps on to the format of the ICKR register. Each bit field is broken out and maps on to the sections of the register as defined in the data sheet.

The final line of code in the above snippet uses the __IO_REG8_BIT macro to map the data structure onto the address 0x50C0 and create a new name with bit level access.

The next thing to note is the while loop which checks the CCOOEN bit in the CCOR register – while (CLK_CCOR_CCOEN != 0);. As above, this uses a three part notation to form a reference, this time it is to a particular bit in a register. This is not a mask as in the previous example. This is broken down as follows:

Name Description
CLK Clock registers are being access.
CCOR This relates to the CCOR register.
CCOEN CCOEN bits in the CCOR register.

Some more digging in the file leads to the following definition:

#define CLK_CCOR_CCOEN           CLK_CCOR_bit.CCOEN

The CLK_CCOR_bit declaration was created by the __IO_REG8_BIT macro. This is the name which has been given to the location in memory of the ICKR register.

CLK_SYSCLKConfig

The next task is to set the prescalar for the system clock. This is being set to 1 to ensure the system runs at 16 MHz. Note that a prescalar of 1 maps to the prescalar bits in the register being set to zero.

CLK_CKDIVR = 0;

CLK_ClockSwitchConfig

The final operation is to switch the system clock to the HSI and this is achieved with the following code:

CLK_SWCR_SWEN = 1;
CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
while (CLK_SWCR_SWBSY != 0);        //  Pause while the clock switch is busy.

Conclusion

So there you have it, three different ways of performing the same task. The method used will be down to individual preference. Happy experimenting.