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Low Power RTOS Demo - ST STM32L
Using IAR and the STM32L-Discovery Board
[RTOS Ports]

STM32L Discovery board from ST for low power applications
The STM32L Discovery Board


The demo application documented on this page demonstrates how the FreeRTOS tick suppression (tickless idle mode) features can be used to minimise the power consumption of an application running on a STM32L152 ARM Cortex-M3 microcontroller from ST. The STM32L is designed specifically for use in applications that require extremely low power consumption.

The demo uses the FreeRTOS IAR ARM Cortex-M3/4 port, the IAR Embedded Workbench for ARM IDE (EWARM), and components of the STM32L Standard Peripheral Library. The project is pre-configured to run on the very low cost STM32L Discovery board.

EWARM Ships with a FreeRTOS Kernel Aware Plug-in

IMPORTANT! Notes on using the STM32L RTOS demo

Please read all the following points before using this RTOS port.

  1. Source Code Organisation
  2. The Demo Application
  3. RTOS Configuration and Usage Details
See also the FAQ My application does not run, what could be wrong?

Source Code Organisation

The official FreeRTOS zip file download contains the source files for all the RTOS ports, and all the demo applications, only a few of which are needed by this project. See the Source Code Organization section for a description of the downloaded files and information on creating a new project.

The IAR project file for the STM32L152 demo application is called RTOSDemo.eww, and is located in the FreeRTOS/Demo/CORTEX_STM32L152_Discovery_IAR directory.

The ST STM32 ARM Cortex-M3 Demo Application

Hardware set up

The demo uses the LED built onto the STM32L Discovery Board and no hardware setup is required.


The single project file can be configured to create either a low power demo, or a standard RTOS demo. The configCREATE_LOW_POWER_DEMO constant is used to switch between the two. configCREATE_LOW_POWER_DEMO is defined at the top of FreeRTOSConfig.h (FreeRTOS/Demo/CORTEX_STM32L152_Discovery_IAR/include/FreeRTOSConfig.h, and included in the IAR project).

Functionality with configCREATE_LOW_POWER_DEMO set to 1

If configCREATE_LOW_POWER_DEMO is set to 1 then main() calls main_low_power(). main_low_power() is implemented in the main_low_power.c C source file.

Low power modes are entered when the RTOS tick is stopped (suppressed). Deeper low power modes have longer wake up periods that lighter low power modes, and power is also used simply entering and especially exiting the low power modes. How the low power modes are used therefore requires careful consideration to ensure power consumption is truly minimised and that the embedded device meets its real time requirements.

The low power demo is configured to select between four different modes depending on the anticipated idle period. Note the time thresholds used to decide which low power mode to enter are purely for convenience of demonstration, and are not intended to represent optimal values for any particular application.

The STM32L specific part of the tickless operation is implemented in the STM32L_low_power_tick_management.c C source file. Tick interrupts are generated from the TIM2 peripheral so a slow input clock can be used and the timer can be configured to carry on running when the STM32 is in the lighter of the used low power modes.


  • Two tasks are created, an Rx task and a Tx task. A queue is created to pass a message from the Tx task to the Rx task.

  • The Rx task blocks on a queue to wait for data, blipping an LED each time data is received (turning it on and then off again) before returning to block on the queue once more.

  • The Tx task repeatedly blocks on an attempt to obtain a semaphore, and unblocks if either the semaphore is received or its block time expires. After leaving the blocked state the Tx task uses the queue to send a value to the Rx task, which in turn causes the Rx task to exit the Blocked state and blip the LED. The rate at which the LED is seen to blip is therefore dependent on the block time.

  • The Tx task's block time is changed by the interrupt service routine that executes when the USER button is pressed. The low power mode entered depends on the block time (as described in the Observed Behaviour section below). Four block times are used: short, medium, long and infinite.

Low Power Behaviour:

  1. The block time used by the Tx task is initialised to its 'short' value, so when the Tx task blocks on the semaphore it times-out quickly, resulting in the LED toggling rapidly. The timeout period is less than the value of configEXPECTED_IDLE_TIME_BEFORE_SLEEP (set in FreeRTOSConfig.h), so the initial state does not suppress the tick interrupt or enter a low power mode.

  2. When the button is pressed the block time used by the Tx task is increased to its 'medium' value. The longer block time results in a slowing of the rate at which the LED toggles. The time the Tx task spends in the blocked state is now greater than configEXPECTED_IDLE_TIME_BEFORE_SLEEP, so the tick is suppressed. The MCU is placed into the 'Sleep' low power state while the tick is suppressed.

  3. When the button is pressed again the block time used by the Tx task is increased to its 'long' value, so the rate at which the LED is observed to blip gets even slow. When the 'long' block time is used the MCU is placed into its 'Low Power Sleep' low power state.

  4. The next time the button is pressed the block time used by the Tx task is set to infinite, so the Tx task does not time out when it attempts to obtain the semaphore, and therefore the LED stops blipping completely. Both tasks are now blocked indefinitely and the MCU is placed into its 'Stop' low power state.

  5. Pressing the button one final time results in the semaphore being 'given' to unblock the Tx task, the CPU clocks being returned to their pre-stop state, and the block time being reset to its 'short' time. The system is then back to its initial condition with the LED blipping rapidly.

Functionality with configCREATE_LOW_POWER_DEMO set to 0

If configCREATE_LOW_POWER_DEMO is set to 0 then main() calls main_full(). main_full() is implemented in the main_full.c C source file.

main_full() creates a comprehensive test and demo application that demonstrates:

The created tasks are from the set of standard demo tasks. Standard demo tasks are used by all RTOS port demo applications. They have no specific functionality, and are created just to demonstrate how to use the FreeRTOS API, and test the RTOS port.

A 'check' software timer is created that periodically inspects the standard demo tasks to ensure all the tasks are functioning as expected. The check software timer's callback function toggles the LED on the STM32L Discovery Board. This gives a visual feedback of the system health. If the LED is toggling every 3 seconds, then the check software timer has not discovered any problems. If the LED is toggling every 200 milliseconds, then the check software timer has discovered a potential problem in at least one task.

Building and executing the demo application

  1. Open FreeRTOS/Demo/CORTEX_STM32L152_Discovery_IAR/RTOSDemo.eww from within the IAR IDE.

  2. Open FreeRTOSConfig.h, and set configCREATE_LOW_POWER_DEMO to generate either the tickless low power demo, or the full test and demo application, as required.

  3. Ensure the target hardware is connected to the host computer using a suitable USB cable.

  4. Press F7 to build the project. The demo should build without any errors or warnings.

  5. After the build completes, press CTRL+D to program the STM32L microcontroller flash memory, start a debug session, and have the debugger break on entry into the main() function.

RTOS Configuration and Usage Details

ARM Cortex-M3 FreeRTOS port specific configuration

Configuration items specific to this demo are contained in FreeRTOS/Demo/CORTEX_STM32L152_Discovery_IAR/include/FreeRTOSConfig.h. The constants defined in this file can be edited to suit your application. In particular -
  • configTICK_RATE_HZ

    This sets the frequency of the RTOS tick interrupt. The setting used by this demo depends on the configCREATE_LOW_POWER_DEMO setting.


    See the RTOS kernel configuration documentation for full information on these configuration constants.


    Whereas configKERNEL_INTERRUPT_PRIORITY and configMAX_SYSCALL_INTERRUPT_PRIORITY are full eight bit shifted values, defined to be used as raw numbers directly in the ARM Cortex-M3 NVIC registers, configLIBRARY_LOWEST_INTERRUPT_PRIORITY and configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY are equivalents that are defined using just the 4 priority bits implemented in the STM32L NVIC. These values are provided because the CMSIS library function NVIC_SetPriority() and STM32 standard peripheral library functions requires the un-shifted 4 bit format.

Attention please!: See the page dedicated to setting interrupt priorities on ARM Cortex-M devices. Remember that ARM Cortex-M cores use numerically low priority numbers to represent HIGH priority interrupts. This can seem counter-intuitive and is easy to forget! If you wish to assign an interrupt a low priority do NOT assign it a priority of 0 (or other low numeric value) as this will result in the interrupt actually having the highest priority in the system - and therefore potentially make your system crash if this priority is above configMAX_SYSCALL_INTERRUPT_PRIORITY. Also, do not leave interrupt priorities unassigned, as by default they will have a priority of 0 and therefore the highest priority possible.

The lowest priority on a ARM Cortex-M core is in fact 255 - however different ARM Cortex-M microcontroller manufacturers implement a different number of priority bits and supply library functions that expect priorities to be specified in different ways. For example, on ST STM32 ARM Cortex-M3 microcontrollers, the lowest priority you can specify is in fact 15 - this is defined by the constant configLIBRARY_LOWEST_INTERRUPT_PRIORITY in FreeRTOSConfig.h. The highest priority that can be assigned is always zero.

NVIC_PriorityGroupConfig( NVIC_PriorityGroup_4 ) must the called before any other interrupt priority related functions from the STM32 Standard peripheral library, as it is in the demo provided.

Each port #defines 'BaseType_t' to equal the most efficient data type for that processor. This port defines BaseType_t to be of type long.

Interrupt service routines

Unlike many FreeRTOS ports, interrupt service routines that cause a context switch have no special requirements, and can be written as per the compiler documentation. The macro portEND_SWITCHING_ISR() can be used to request a context switch from within an interrupt service routine.

Note that portEND_SWITCHING_ISR() will leave interrupts enabled.

The following source code snippet is provided as an example. The interrupt uses a semaphore to synchronise with a task (not shown), and calls portEND_SWITCHING_ISR() to ensure the interrupt returns directly to the task if the task has an equal or higher priority than the interrupted task. See the function EXTI0_IRQHandler() in the file main_low_power.c included in this demo project for another example.

void Dummy_IRQHandler(void)
long lHigherPriorityTaskWoken = pdFALSE;

    /* Clear the interrupt if necessary. */

    /* This interrupt does nothing more than demonstrate how to synchronise a
    task with an interrupt.  A semaphore is used for this purpose.  Note
    lHigherPriorityTaskWoken is initialised to zero. */
    xSemaphoreGiveFromISR( xTestSemaphore, &lHigherPriorityTaskWoken );

    /* If there was a task that was blocked on the semaphore, and giving the
    semaphore caused the task to unblock, and the unblocked task has a priority
    higher than the current Running state task (the task that this interrupt
    interrupted), then lHigherPriorityTaskWoken will have been set to pdTRUE
    internally within xSemaphoreGiveFromISR().  Passing pdTRUE into the
    portEND_SWITCHING_ISR() macro will result in a context switch being pended to
    ensure this interrupt returns directly to the unblocked, higher priority,
    task.  Passing pdFALSE into portEND_SWITCHING_ISR() has no effect. */
    portEND_SWITCHING_ISR( lHigherPriorityTaskWoken );

Only FreeRTOS API functions that end in "FromISR" can be called from an interrupt service routine - and then only if the priority of the interrupt is less than or equal to that set by the configMAX_SYSCALL_INTERRUPT_PRIORITY configuration constant (or configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY).

Resources used by FreeRTOS

When configCREATE_LOW_POWER_DEMO is set to 0 the standard FreeRTOS Cortex-M port is used, which requires exclusive use of the SysTick and PendSV interrupts. SVC number #0 is also used.

When configCREATE_LOW_POWER_DEMO is set to 1 exclusive access to the TIM2 peripheral is required.

Memory allocation

Source/Portable/MemMang/heap_4.c is included in the ARM Cortex-M3 demo application project to provide the memory allocation required by the RTOS kernel. Please refer to the Memory Management section of the API documentation for full information.


Note that vPortEndScheduler() has not been implemented.

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