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Microchip ARM Cortex-M4F CEC1302
Low Power RTOS Demo - Using the Keil, GCC and MikroC development tools
[RTOS Ports]


Introduction

This page documents an RTOS demo application for the ARM Cortex-M4F based CEC1302 microcontroller from Microchip. Pre-configured projects are provided for the following compiler and tool combinations:
  • Keil uVision, using the ARM compiler
  • Keil uVision, using the GCC compiler
  • mikroC Pro for ARM

The project can be configured to create either a simple blinky style project that demonstrates how power can be saved by using the RTOS's tickless idle feature, or a more comprehensive test and demo application that tests the port's integrity and demonstrates many RTOS features.



IMPORTANT! Notes on using the FreeRTOS CEC1302 demo project

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?, noting in particular the recommendation to develop with configASSERT() defined in FreeRTOSConfig.h.



Source Code Organisation

The FreeRTOS zip file contains the source files for all the FreeRTOS ports, and all the demo applications. Only a few of these files 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 Keil uVision project that uses the ARM compiler is called RTOSDemo.uvprojx, and is located in the FreeRTOS/Demo/CORTEX_M4F_CEC1302_Keil_GCC/Keil_Specific directory of the main FreeRTOS download.

  • The Keil uVision project that uses the GCC compiler is also called RTOSDemo.uvprojx, and is located in the FreeRTOS/Demo/CORTEX_M4F_CEC1302_Keil_GCC/GCC_Specific directory of the main FreeRTOS download.

  • The MikroC Pro for ARM project is called RTOSDemo.mcpar, and is located in the FreeRTOS/Demo/CORTEX_M4F_CEC1302_MikroC/MikroC_Specific directory of the main FreeRTOS download.
Caution: The MikroC compiler will leave many temporary files throughout your project directory structure, the built in editor is not compatible with the formatting conventions used by the RTOS source files, and to allow the FreeRTOS source files to be built the FreeRTOS MikroC port layer #defines the 'const' keyword away to nothing.



The Microchip ARM Cortex-M4 RTOS Demo Application


Hardware set up - uVision projects

Normally RTOS demo applications indicate their status by controlling the rate at which an LED is toggled. However, the uVision demos do not target any specific hardware, so are not configured to toggle any specific LED on any particular IO port. Instead, a variable called ulLED is incremented each time an LED would otherwise be toggled.

The ulLED variable is incremented by the macro configTOGGLE_LED(), which is defined in FreeRTOSConfig.h. If your target hardware has a spare LED, then configTOGGLE_LED() can be updated to make use of the LED. Otherwise ulValue can be added to the uVision IDE's data watch window, where its value will be seen to updated as the RTOS demo application executes (provided a capable debug interface is used, such as a ULINK).

The ARM compiler and GCC compiler versions of the uVision projects can also be executed on the Clicker hardware described in the next section by creating an adapter cable that maps the Mikro mProg cable to a standard ARM JTAG connector. The required pin-out is detailed in the table below (assumes a 20-pin JTAG connector).

Signal Names ARM 20 Pin # mPROG Pin #
VCC +3.3V 1 1
Ground 20 9
JTAG_TDI 5 8
JTAG_TMS 7 2
JTAG_TCLK 9 4
JTAG_TDO 13 6
RTCK 11 and 12 Ground


Hardware set up - MikroC Pro for ARM project

The MikroC project is pre-configured to run on a CEC1302 Clicker board, and uses the LED built onto that hardware.


Functionality

The project provides two RTOS demo applications. A simple blinky style project that demonstrates low power tickless functionality, and a more comprehensive test and demo application. The configCREATE_LOW_POWER_DEMO setting, which is defined in FreeRTOSConfig.h, is used to select between the two.


Functionality with configCREATE_LOW_POWER_DEMO set to 1

When configCREATE_LOW_POWER_DEMO is set to 1 main() calls main_low_power(), which creates a very simple demo as follows:
  • Two tasks are created, an Rx task and a Tx task.

  • The Rx task blocks on a queue to wait for data, blipping (turning on then off again) an LED (or toggling the ulLED variable) each time data is received. The LED is only blipped on briefly so it does not effect current consumption too much.

  • The Tx task sends a value through the queue to the Rx task every 1000ms (resulting in the Rx task exiting the blocked state to blip the LED).


Observed behaviour when configCREATE_LOW_POWER_DEMO is set to 1

Both tasks spend most of their time in the Blocked state, during which periods the RTOS tick is turned off, and the ARM Cortex-M4F is placed into a low power state. Every 1000ms the MCU will come out of the low power state, turn the LED on, return to the low power state for 10ms, before leaving the low power state again to turn the LED off. This will be observed as a short blip on the LED every 1000ms.


RTOS implementation when configCREATE_LOW_POWER_DEMO is set to 1

The demo is provided with a low power tickless implementation that uses the CEC1302 hibernation timer to generate the RTOS tick interrupt.

This CEC1302 specific tickless implementation includes the same configPRE_SLEEP_PROCESSING() and configPOST_SLEEP_PROCESSING() macros that are described on the page that documents the generic Cortex-M tickless implementation. The pre sleep macro can be defined to take additional application specific actions to improve energy efficiency before entering the low power state, and the post sleep macro can be defined to reverse any of the actions taken by the pre sleep macro. For example, if the pre-sleep macro is defined to turn a peripheral off, and the post sleep macro is defined to turn the peripheral on again, then the peripheral will automatically be turned off prior to each entry into the sleep mode, and automatically turned back on again on each exit from the sleep mode.


Functionality with configCREATE_LOW_POWER_DEMO set to 0

When configCREATE_LOW_POWER_DEMO is set to 0 main() calls main_full(), which 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 FreeRTOS 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' task is created that periodically inspects the standard demo tasks to ensure all the tasks are functioning as expected. The check task toggles an LED (or increments the ulLED variable). This gives a visual feedback of the system health. If an LED is toggling every 3 seconds, then the check task has not discovered any problems. If the LED is toggling every 200 milliseconds, then the check task has discovered a problem in one or more tasks.


Building and executing the demo application

Using the uVision project with the GCC compiler

  1. Ensure a suitable GCC compiler is installed on the host computer. The project was created and tested with the arm-none-eabi-gcc compiler, pre-built binaries for which are freely available from the launchpad.net website.

  2. Open FreeRTOS/Demo/CORTEX_M4F_CEC1302_Keil_GCC/GCC_Specific/RTOSDemo.uvprojx from within the Keil IDE.

  3. Open the Manage Project Items window within the uVision IDE ("Project->Manage->Project Items" menu item) and set the GCC prefix and installation folder to be correct for your host computer.

    setting the path to the GCC compiler in Keil uVision
    Setting the path to the GCC compiler binary
    Note the path does not include the training "/bin"

  4. 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.

  5. Ensure the target hardware is connected to the host computer through a suitable debug connector, such as a ULINK2.

  6. Select 'Rebuilt All Target Files' from the IDE's 'Project' menu, the RTOSDemo project should compile without any errors or warnings.

  7. After the build completes, select "Start/STOP Debug Session" from the IDE's Debug menu to download the application to the CEC1302 ARM Cortex-M4F microcontroller RAM memory, start a debug session, and have the debugger break on entry into the main() function. If the debug session does not start as expected then ensure the debug options, and in particular the "Connect & Reset Options" section of the debug options, are set as shown below.

    configuring a debug session to connect to the ARM Cortex-M processor
    The required connect and reset options


Using the uVision project with the ARM compiler

  1. Open FreeRTOS/Demo/CORTEX_M4F_CEC1302_Keil_GCC/Keil_Specific/RTOSDemo.uvprojx from within the Keil 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 through a suitable debug connector, such as a ULINK2.

  4. Select 'Rebuilt All Target Files' from the IDE's 'Project' menu, the RTOSDemo project should compile without any errors or warnings, although a warning may be generated by the linker as, to enable the maximum number of tests to be added to the comprehensive demo, the linker script is configured to place both code and data in the same named section.

  5. After the build completes, select "Start/STOP Debug Session" from the IDE's Debug menu to download the application to the CEC1302 ARM Cortex-M4F microcontroller RAM memory, start a debug session, and have the debugger break on entry into the main() function. If the debug session does not start as expected then ensure the debug options, and in particular the "Connect & Reset Options" section of the debug options, are set as shown by the image provided as part of the GCC instructions above.


Using the MikroC Pro compiler for ARM and the Clicker hardware

Caution: The MikroC compiler will leave many temporary files throughout your project directory structure, the built in editor is not compatible with the formatting conventions used by the RTOS source files, and to allow the FreeRTOS source files to be built the FreeRTOS MikroC port layer #defines the 'const' keyword away to nothing.
  1. Open FreeRTOS/Demo/CORTEX_M4F_CEC1302_MikroC/MikroC_Specific/RTOSDemo.mcpar from within the MikroC Pro IDE.

  2. Open the Search Paths window ("Projects->Edit Search Paths" menu item) and update the paths to the source files and header files to be correct for your host computer.

    setting the path to the free RTOS source files and header files
    Setting the path to source and header files

  3. As provided, the MikroC demo uses the FreeRTOS/Source/portable/MemMang/heap_4.c memory allocator, and a malloc failed hook is defined. It is necessary to make a small edit to the heap_4.c source file to make it compatible with the MikroC compiler. Open heap_4.c and locate the following code:
    #if( configUSE_MALLOC_FAILED_HOOK == 1 )
    {
        if( pvReturn == NULL )
        {
            extern void vApplicationMallocFailedHook( void ); /* Move this line. */
            vApplicationMallocFailedHook();
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }
    }
    #endif
    		
    Move the line highlighted (the line starting 'extern') from its current position to the top of the source file so it has file scope rather than block scope.

  4. 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.

  5. Select 'Build' from the IDE's 'Build' menu to create an executable image.

  6. Ensure the Clicker hardware is powered through its USB port, and connected using a MikroProg flash programmer, before selecting "mE Programmer" from the "Tools" menu to download the executable image to the CEC1302 ARM Cortex-M4F microcontroller. If the download does not start as expected then ensure the Programmer/Debugger Options are as shown in the image below ("Tools->Programmer/Debugger Options" menu item).

    setting the MikroC Pro compiler debug options for the ARM Cortex-M processor
    The required programmer and debug options

  7. After the download completes, select "Start Debugger" from the "Debug" menu to start a debug session, and run the application. If the debugger does not start as expected then ensure the Programmer/Debugger Options are as shown in the image above ("Tools->Programmer/Debugger Options" menu item).



RTOS Configuration and Usage Details


ARM Cortex-M4 FreeRTOS port specific configuration

Configuration items specific to this demo are contained in /FreeRTOS/Demo/CORTEX_M4F_CEC1302_Keil_GCC/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.

  • configKERNEL_INTERRUPT_PRIORITY and configMAX_SYSCALL_INTERRUPT_PRIORITY

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

  • configLIBRARY_LOWEST_INTERRUPT_PRIORITY and configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY

    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-M4 NVIC registers, configLIBRARY_LOWEST_INTERRUPT_PRIORITY and configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY are equivalents that are defined using just the 3 priority bits implemented in the CEC1302 NVIC. These values are provided because the CMSIS library function NVIC_SetPriority() requires the un-shifted 3 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 Microchip CEC1302 ARM Cortex-M4 microcontrollers, the lowest priority you can specify is in fact 7 - this is defined by the constant configLIBRARY_LOWEST_INTERRUPT_PRIORITY in FreeRTOSConfig.h. The highest priority that can be assigned is always zero.

It is also recommended to ensure that all priority bits are assigned as being preemption priority bits, and none as sub priority bits.

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.


Aggregated and disaggregated interrupts

The CEC1302 can route groups of interrupt sources to the same interrupt vector (aggregated interrupts), or route all interrupt sources to their own unique interrupt vector (disaggregated interrupts). The RTOS itself only uses ARM Cortex-M system interrupts, which are always routed to the standard ARM Cortex-M system vectors, but the demo application also makes use of btimer (basic timer) and htimer (hibernation timer) interrupts, both of which can be aggregated or disaggregated. In order to demonstrate both methods, the tickless low power demo is configured to use aggregated interrupts and the comprehensive demo is configured to use disaggregated interrupts (note the comprehensive demo uses disaggregated interrupts as the interrupt nesting test cannot function if all the timer interrupts are routed through the same vector).


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 portYIELD_FROM_ISR() can be used to request a context switch from within an interrupt service routine.

Note that portYIELD_FROM_ISR() will leave interrupts enabled.

The following source code snippet is provided as an example. The interrupt uses a direct to task notification to synchronise with a task (not shown), and calls portYIELD_FROM_ISR() to ensure the interrupt returns directly to the task if the task has an equal or higher priority than the interrupted task.

void Dummy_IRQHandler(void)
{
long lHigherPriorityTaskWoken = pdFALSE;

    /* Clear the interrupt if necessary. */
    Dummy_ClearITPendingBit();

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

    /* If the notified task was blocked waiting for the notification, and the
    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 vTaskNotifyGiveFromISR().  Passing
    pdTRUE into the portYIELD_FROM_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 portYIELD_FROM_ISR()
    has no effect. */
    portYIELD_FROM_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

FreeRTOS requires exclusive use of the SysTick and PendSV interrupts. SVC number #0 is also used.


Switching between the pre-emptive and co-operative RTOS kernels

Set the definition configUSE_PREEMPTION within FreeRTOSConfig.h to 1 to use pre-emption or 0 to use co-operative. The full demo application may not execute correctly when the co-operative RTOS scheduler is selected.


Compiler options

As with all the ports, it is essential that the correct compiler options are used. The best way to ensure this is to base your application on the provided demo application files.


Memory allocation

configSUPPORT_STATIC_ALLOCATION and configSUPPORT_DYNAMIC_ALLOCATION are both set to 1, allowing RTOS objects to be created statically or dynamically. Source/Portable/MemMang/heap_4.c is used to provide the RAM required by dynamically allocated RTOS objects. Please refer to the Memory Management section of the API documentation for full information.


Miscellaneous

Note that vPortEndScheduler() has not been implemented.





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