Embedded Developer

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

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. f_master). The prescalar divides the f_master 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:

f_counter = f_master / 2^TIM2_PSCR

where f_counter 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:

f_counter = f_master / 2^TIM2_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:

f_interrupt = f_counter / counter

Putting the two together we get the following:

f_interrupt = f_master / (2^TIM2_PSCR * counter)

A little rearranging gives:

(2^TIM2_PSCR * counter) = f_master / f_interrupt

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

(2^TIM2_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

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

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:

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 f_master 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:

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:

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:

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

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

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 V_ss and V_dd (pins 7 and 9) and a 1uF between V_ss and V_cap (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

I have been using the IAR development environment to develop code for the STM8S chip for several months now. One of the things that has been a little bit of an annoyance is the need to keep reconfiguring each new project as it is created. One option would be to create a template project and keep copying the template and just rename the project files and add it to a new workspace. There must be a better way…

This is the thought which started me delving into the way in which IAR stores it’s templates and asking if I could make my own.

Where are the Templates

So the first thing to do is to work out if the templates are stored as files and if so where they are. Breaking out Explorer I found an interesting directory which looked like it might be a candidate, namely:

C:Program Files (x86)\IAR Systems\Embedded Workbench 6.0 Kickstartstm\configtemplateproject

I remembered that when you create a new IAR project you are presented with three options:

1. asm
2. C
3. C++

Guess what, in that directory I found three directories and three files with the extension .projtempl. For each project name in the default projects (asm, C, C++) there was a matching directory and matching file. We may be in luck.

What’s in the Files?

The .projtempl files are pretty small so I guessed they were text files of some sort. Feeding them to Notepad++ confirmed that they were indeed text files. In fact they were XML files. In fact the default C file looks like this:

<?xml version="1.0" encoding="iso-8859-1"?>

<template>
  <displayname>C</displayname>
    <description>Creates a C project.
    </description>
  <files>
    <file>$PROJ_DIR$main.c</file>
  </files>
</template>

So my guess is that the displayname and description fields determine what is shown to the user and the files section tells the environment what files are to be included in the project.

What’s in the Directory?

Next port of call is the directory for the project. So continuing with the C project type we find that the C directory contains two files:

1. main.c
2. templproj.ewp

Looking inside the main.c file we find the default code which is placed inside the main.c file which is created for you when you ask for a new C project.

Putting it all together

If we put all of the pieces together, I think that we can create out own project templates using the default C project as an example. Note that I am using the C project type as an example as it is the only high level language version supported by the Kickstart edition of the IAR programming environment for the STM8S processor.

So I think we need to do the following in order to create our own template:

1. Copy the default C directory and templproj file and give them a consistent name (i.e. the name of the directory matches the root of the file name)
2. Edit the templproj file and change the displayname, the description and optionally the files section to match our new project type
3. Add files to the new directory and edit any existing files as required

So let’s give it a go and see what happens.

Creating a New Template

So the first thing to do is to decide on the name of the template we are creating and copy the files. I decided on the imaginative name of My Default C Project for the new template. Catchy isn’t it.

One of the first problems I encountered was the fact that this is a system directory as far as Windows 7 is concerned and so I had to modify the permissions. If you hit any problems with this step of any others you need to make sure that you change the permissions to allow editing and creating files. I came across this on a few occasions. If you don’t know how to do this then Google your OS to find out how to change permissions. I gave Full Control access to myself for the duration of the task and changed the permissions back afterwards.

So at this point I have a My Default C Project.templproj file and a My Default C Project directory. The directory contains the same files as the C directory.

Next step, edit the templproj file and change the displayname and the description entries. I made the displayname match the directory and project file names. The description field expands on this a little more explaining what is included in the project.

Moving on to the main.c file in our new directory I edited this to contain a basic C program which included some basic headers and reset the system clock:

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

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

So What Does This Do?

Next step, fire up IAR and create a new project. Guess what….

IT’S THERE !

Selecting the new project prompts me to create a new workspace (which I do) and then opens up the main.c file. Sure enough, it matches the code I have entered into the new template.

What’s Next?

Next step is to try and edit the templproj.ewp file in the C directory and set up some of the default options. For instance, I always use ST-Link/V2 to deploy to the STM8S chip. Maybe try adding a few files to the directory and the .projtempl file. The rest, as they say, is left as an exercise for the reader.

This post continues the low level use of the STM8S registers and shows you how to configure the STM8S system clock. In particular we will configure the system clock to use the internal high speed clock.

So let us start with a little background. The STM8S can be driven by three clock sources:

• High Speed Internal (HSI) RC oscillator running at 16MHz
• Low Speed Internal (LSI) RC oscillator running at 128KHz
• External clock (1-24 MHz) which can be either an external crystal or user supplied clock.

We will be using the internal oscillator as this will be satisfactory for our purposes. An external clock may be more accurate but for most of the work I am doing I am happy to sacrifice the accuracy for the lower component count and greater simplicity of my designs.

At start up, the STM8S will be using HSI with a prescalar to bring the initial clock speed down to 2MHz. We will be resetting the clock prescalar to allow the chip to run at the full 16MHz.

The switching algorithm we will use is as follows:

1. Reset all of the clock registers to the power on state
2. Select the clock we wish to use
3. Enable the switching mechanism
4. Wait until the STM8S indicates that the clock has stabilised

Clock Registers

The description of the clock registers start around page 89 of the STM8S Reference Manual. In this article we will run through the registers concentrating on the values we will be setting. For a fuller description you should refer to the STM8S Reference Manual.

CLK_ICKR – Internal Clock Register

The only bits we will really be interested in for this example bits 1 and 0, this allows us to select the HSI and see if it is ready.

Our example here will be resetting the clock source on the STM8S chip to use the internal oscillator using HSIEN and using the HSIRDY bit to determine when the clock has been setup and is stable.

CLK_ECKR – External Clock Register

This register is used to provide information about the state of the external clock. We will not be using this register for anything other than resetting the register to the power on state.

CLK_CMSR – Clock Master Status Register

This register is set and cleared by hardware and indicates which clock source is currently selected as the master clock source.

CLK_SWR – Clock Master Switch Register

This register is written to by the user program and is used to select the master clock source. The register cannot be written to whilst a clock source switch is on-going. You can test for the switch using the SWBSY in the CLK_SWCR register.

Clock Switch Control Register – CLK_SWCR

The Clock Switch Control Register is used to control how and when the clock switch is performed.

In our case we will be using this register to start the clock switch process.

CLK_CKDIVR – Clock Divider Register

The system clock can be divided down to allow for lower clock frequencies to be used. This register allows for the HSI prescalar (divider) and the CPU prescalar to be set.

The following values can be used for the HSI prescalar:

The following values are allowed for the CPU prescalar:

In this case we will want the CPU to be running at full speed and so no prescalars will be applied to either the master clock or the HSI.

CLK_PCKENR1 & CLK_ PCKENR2 – Peripheral Clock Gating Registers

These two registers enable (set to 1 and disable (set to 0) the clocks used to control the peripherals used in the application. I have fallen foul of these values a few times. If you set these registers or zero then you will turn off the peripheral clocks. This has the effect of turning off the feature you may be trying to use. If in doubt set both of these registers to 0xff. You can always refine the values later by restricting the clock signals to only those which you are using.

Enabling the service will use the frequency of the master clock. Disabling the clock to the service will turn it off.

The <iosstm8s103f3.h> file does not contain definitions for these bits, you need to define your own constants for these. I have recently taken to just turning them all on as I’m not too worried about the power requirements for the applications I am working on.

CLK_CSSR – Clock Security System Register

We will not be using this register in this example apart for setting the default power on value. Please see the STM8S Reference Manual for more information.

CLK_CCOR – Configurable Clock Output Register

We will not be using this register in this example apart for setting the default power on value. Please see the STM8S Reference Manual or future examples for more information.

CLK_HSITRIMR – HSI Clock Calibration Trimming Register

We will not be using this register in this example apart for setting the default power on value. Please see the STM8S Reference Manual for more information.

CLK_SWIMCCR – SWIM Clock Control Register

We will not be using this register in this example apart for setting the default power on value. Please see the STM8S Reference Manual for more information.

Software

As previously mentioned, this example will simply set the system clock to the maximum speed the HSI can deliver. We will then toggle a GPIO pin and hook this up to a scope to get an idea of the output. We will never see the full 16MHz clock speed of the HSI but we should be able to see that the output frequency changes when we change the clock dividers.

So first thing, let’s get the system set up and running at the full 16 MHz and see what we the scope produces.

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

//
//  Main program loop.
//
int main(void)
{
    __disable_interrupt();
    //
    //  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 up to 10 MHz.
    //
    //  Now setup the system clock
    //
    InitialiseSystemClock();
    __enable_interrupt();
    while (1)
    {
        PD_ODR_ODR4 = 1;    // Turn Port D, Pin 4 on.
        PD_ODR_ODR4 = 0;    // Turn Port D, Pin 4 off.
    }
}

Running this code resulted in the following trace on the oscilloscope:

GPIO Output when the System Clock is set to HSI with no Divider.

The frequency of the out was 2.693Mhz.

So now let’s see what happens when we set the prescalar for HSI to be 2 (i.e. half the clock speed. To do this we need to edit the code slightly, so change this:

CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.

to this:

CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
CLK_CKDIVR_HSIDEV = 1;				//	Set the HSI divider to 2.

The result is the following trace:

GPIO Output when System Clock is set to HSI divided by 2

This looks remarkably like the trace above but the oscilloscope shows that the frequency of the signal is now 1.335MHz, or approximately half of the frequency of the first trace.

The full project for this article can be downloaded from here. This code has been tested with my reference platform, the Variable Labs Protomodule and the STM8S Discovery board.

Compatibility

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

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 V_SS and V_CAP and a 100 nF capacitor is placed between V_DD and V_SS. 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

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:

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:

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

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