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ST STM32 Primer ARM Cortex-M3 Demo
This page describes the FreeRTOS demo application for the STM32 Primer – a novel evaluation platform for the STMicroelectronics STM32 ARM Cortex-M3 microcontroller. The demo uses the GCC compiler with the Raisonance Ride V7 IDE.
The demo utilises drivers and other source files from CircleOS (which, unlike FreeRTOS.org, is not a real time kernel). These files are licensed separately from FreeRTOS.org. Users must familiarise themselves with the CircleOS license. Please note that the FreeRTOS demo is not itself a CircleOS application and will overwrite CircleOS on the STM32 Primer. The batch files located in the [Program Files]RaisonanceRideLibARMCircleOS directory of your Raisonance Ride distribution can be used to restore CircleOS to the STM32 Primer hardware.
Using RIDE version 7: FreeRTOS V5.1.1 will not build with the latest versions of the RIDE libraries. The head revision in the FreeRTOS SVN repository has already been fixed – the necessary changes will be included in the next release.
Upgrading to FreeRTOS V5.0.3: FreeRTOS V5.0.3 introduced the configMAX_SYSCALL_INTERRUPT_PRIORITY configuration option to the ARM Cortex-M3 port. See the RTOS kernel configuration documentation for full information on this feature.
Upgrading to FreeRTOS V4.8.0: Prior to V4.8.0 the FreeRTOS kernel did not make use of the SVCall interrupt. From V4.8.0 onwards it does. Therefore, to upgrade an older project to the V4.8.0 standard, a small edit to the startup code is required. To do this, simply install vPortSVCHandler() in the SVCall position within the interrupt vector table (contained in the startup source file). The demo projects included in the FreeRTOS download have already been updated so these can be used as an example.
IMPORTANT! Notes on using the STM32 Primer ARM Cortex-M3 DemoPlease read all the following points before using this RTOS port.
My application does not run, what could be wrong?
Source Code OrganisationThe FreeRTOS download includes the source code for all the FreeRTOS ports and therefore contains many more files than are required for this demo. See the Source Code Organization section for a description of the downloaded files and information on creating a new project.
The Ride workspace file for the STM32F103 Primer Ride demo is called RTOSDemo.rprj and is located in the FreeRTOS/Demo/CORTEX_STM32F103_Primer_GCC directory.
The Demo Application
Demo application hardware setupThe demo application uses the LED and display built onto the evaluation board so no specific hardware setup is required.
A USB interface is used to connect directly between the STM32 Primer and the host PC.
Building and running the demo applicationConnect the USB port marked “Debug” on the STM32 Primer to the host PC.
- Open the FreeRTOS/Demo/CORTEX_STM32F103_Primer_GCC/RTOSDemo.rprj project from within the Ride IDE.
- Select ‘Make Project’ from the IDE ‘Project’ menu. The project should build with no errors or warnings.
- Select ‘Start’ from the IDE ‘Debug’ menu. The microcontroller flash memory will be programmed with the newly built binary and the debugger will break on the entry to main().
FunctionalityThe demo application creates 22 real time tasks. These tasks consist predominantly of the standard demo application tasks (see the demo application section for details of the individual tasks).
The following tasks and tests are created in addition to the standard demo tasks:
- High priority interrupt test
A high frequency periodic interrupt is generated using a free running timer to demonstrate the use of the ‘configKERNEL_INTERRUPT_PRIORITY’ configuration constant. The interrupt service routine measures the number of processor clocks that occur between each interrupt – and in so doing measures the jitter in the interrupt timing. The maximum measured jitter time is latched in the ulMaxJitter variable, and displayed on the LCD display by the ‘Check’ task as described below. The fast interrupt is configured and handled in the timertest.c source file. This demonstrates how the RTOS kernel can be configured so as to have no impact on higher priority interrupt processing.
The ARM Cortex-M3 core has the ability to hasten the entry into an interrupt service routine (and therefore reduce latency) by up to 8 cycles should a high priority interrupt occur while a lower priority interrupt is already being serviced. The measured jitter time should therefore be no more than 8 clock cycles.
- LCD task
The LCD task is a ‘gatekeeper’ task. It is the only task that is permitted to access the display directly. Other tasks wishing to write a message to the LCD send the message on a queue to the LCD task instead of accessing the LCD themselves. The LCD task just blocks on the queue waiting for messages – waking when messages arrive.
Two types of message are received by the LCD task. The first type contain strings for display on the LCD. The second type contains an instruction to update the LCD in accordance with the current MEMS input as described below (the MEMS functionality is from the CircleOS demo).
- Check task
This only executes every five seconds but has the highest priority so is guaranteed to get processor time. Its main function is to check that all the standard demo tasks are still operational. Should any unexpected behaviour within a demo task be discovered the ‘check’ task will write an error to the LCD (via the LCD task). If all the demo tasks are executing with their expected behaviour then the check task writes PASS along with the max jitter time to the LCD (again via the LCD task), as described above.
- Tick hook
This is a basic Tick Hook that periodically sends a message to the LCD task to request that the MEMS input be updated.
- Flash task
This is a very basic task that does nothing but wake every second, then flash an LED. It is used for timing validation only.
When executing correctly the demo application will behave as follows:
- The green LED is under control of the ‘flash’ task and will toggle every second.
- The MEMS input is used to control the position of a little ball on the LCD. The ball can be moved about the LCD by tilting the STM32 Primer. 45% is set as the neutral position. This functionality is taken from the CircleOS demo, but in this case executed from within its own autonomous task.
- The ‘check’ task will write “PASS” and the jitter time in nanoseconds to the display every 5 seconds.
RTOS port specific configurationConfiguration items specific to these demos are contained in FreeRTOS/Demo/CORTEX_STM32F103_Primer_GCC/FreeRTOSConfig.h. The constants defined in this file can be edited to suit your application. In particular –
This sets the frequency of the RTOS tick. The supplied value of 1000Hz is useful for testing the RTOS kernel functionality but is faster than most applications require. Lowering this value will improve efficiency.
- configKERNEL_INTERRUPT_PRIORITY and configMAX_SYSCALL_INTERRUPT_PRIORITY
See the RTOS kernel configuration documentation for full information on these configuration constants.
Attention please!: Remember that ARM Cortex-M3 cores use numerically low priority numbers to represent HIGH priority interrupts, which 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 can 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.
The lowest priority on a ARM Cortex-M3 core is in fact 255 – however different ARM Cortex-M3 vendors implement a different number of priority bits and supply library functions that expect priorities to be specified in different ways. For example, on the STM32 the lowest priority you can specify in an ST driver library call is in fact 15 – and the highest priority you can specify is 0. This is defined by the constant configLIBRARY_KERNEL_INTERRUPT_PRIORITY in FreeRTOSConfig.h.
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.
Note that vPortEndScheduler() has not been implemented.
Interrupt service routinesUnlike most 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 ISR. The interrupt driven UART demo in the STM32/IAR demo can be used as an example. See the file FreeRTOS/Demo/CORTEX_STM32F103_IAR/serial/serial.c for a full example, but note that this example is intended to demonstrate the mechanisms required only and should not be used as an example of an optimal UART driver.
Note that portEND_SWITCHING_ISR() will leave interrupts enabled.