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Low Power RTOS Demo - Atmel SAM4L
Using the free Atmel Studio 6 IDE, GCC, and the Atmel Software Framework
[RTOS Ports]


SAM4L-EK microcontroller evaluation kit from Atmel

Introduction

The application documented on this page demonstrates how the FreeRTOS tick suppression features can be used to minimise the power consumption of an application running on a SAM4L ARM Cortex-M4 microcontroller from Atmel. The SAM4L is designed specifically for use in applications that require extremely low power consumption.

The demo uses the FreeRTOS GCC ARM Cortex-M3/4 port, the free Atmel Studio 6 IDE, and components of the comprehensive Atmel Software Framework (ASF). The project is pre-configured to run on the SAM4L-EK evaluation kit.


IMPORTANT! Notes on using the FreeRTOS SAM4L 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?

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 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 Atmel Studio 6 Solution file for the ATSAM4L demo application is called FreeRTOS_Demo.atsln, and is located in the FreeRTOS/Demo/CORTEX_M4_ATSAM4L_Atmel_Studio directory.



The Atmel ARM Cortex-M4 Demo Application


Hardware set up

The demo uses the LEDs built onto the SAM4L-EK and no hardware setup is required.


Functionality

configCREATE_LOW_POWER_DEMO is a constant that is defined at the top of FreeRTOSConfig.h. The low power demo is created when the project is built with configCREATE_LOW_POWER_DEMO set to 1. A standard kernel only demo (without tick suppression) is created when the project is built with configCREATE_LOW_POWER_DEMO set to 0.


Functionality with configCREATE_LOW_POWER_DEMO set to 1

When configCREATE_LOW_POWER_DEMO is set to 1 main() calls main_low_power(). main_low_power() 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 an LED each time data is received (turning it on and then off again) before returning to block on the queue once more. The LED is only blipped on briefly so it does not effect the current reading too much.

  • The Tx task repeatedly enters the Blocked state for 500ms. On exiting the blocked state the Tx task sends a value through the queue to the Rx task (causing the Rx task to exit the blocked state and blip the LED).


Observed behaviour when configCREATE_LOW_POWER_DEMO is set to 1

For optimum low power results the SAM4L-EK must be connected and powered using only the JTAG USB connector, but the debugger must not be connected (the application must be executed 'stand alone').

The MCU spends most of its time in the Retention low power state (see the implementation section below), during which times the current reading displayed on the board monitor that is built onto the SAM4L-EK will show a low current reading.

Every 500ms the MCU will come out of the low power state to turn the LED on, then return to the low power state for 20ms, before leaving the low power state again to turn the LED off. This will be observed as a rapid blip on the LED every 500ms, and two very brief dots appearing on the graph of current consumption that is displayed by the board monitor that is built onto the SAM4L-EK. The two dots will often be observed as a single dot due to their speed.


Current readings as shown by the board monitor display when the
tickless low power application is executing


RTOS implementation when configCREATE_LOW_POWER_DEMO is set to 1

The SAM4L microcontroller has several different low power modes. The low power mode that consumes the least energy without loosing the contents of the static RAM and the CPU registers is called Retention mode. The RTOS turns off the tick interrupt and places the SAM4L into its Retention low power state whenever both tasks are in the Blocked state (when only the Idle task is able to execute). Turning the tick interrupt off allows the microcontroller to remain in Retention mode until it is time for a task to leave the Blocked state and execute. If the RTOS tick interrupt was not suppressed in this way the microcontroller would have to exit Retention mode each time a tick interrupt occurred.

The standard RTOS Cortex-M port uses the Cortex-M SysTick timer to generate the RTOS tick interrupt, but the SysTick timer is halted when the SAM4L enters retention mode so it cannot be used in this demo. Instead the RTOS tick is generated by the Asynchronous Timer (AST). The AST continues running in Retention mode, and its full 32-bit width and slow clocking allows the RTOS tick to be suppressed for vastly longer periods than possible when using SysTick.


Functionality with configCREATE_LOW_POWER_DEMO set to 0

When configCREATE_LOW_POWER_DEMO is set to 0 main() calls main_full(). 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 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' 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 an LED on the SAM4L-EK hardware. This gives a visual feedback of the system health. If an 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 problem in one or more tasks.


Building and executing the demo application

  1. Open FreeRTOS/Demo/CORTEX_M4_ATSAM4L_Atmel_Studio/FreeRTOS_Demo.atsln from within the Atmel Studio 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 the J-Link USB connector built onto the SAM4L-EK board.

  4. Select 'Build Solution' from the IDE's 'Build' menu, the RTOSDemo project should build without any errors or warnings.

  5. After the build completes, select "Start Debug and Break" from the IDE's Debug menu to program the SAM4L microcontroller flash memory, start a debug session, and have the debugger break on entry into the main() function.

    Two things can prevent the download:

    1. If the SAM4L-EK has not been connected before the J-Link driver might open a window to ask if you want to update the J-Link firmware - but the window (at least at the time of writing) will appear underneath the Atmel Studio 6 window.

    2. The J-Link will default to using JTAG mode, but the SAM4L-EK requires SWD mode as shown below:


      Ensure to use SWD mode

  6. Ensure the debugger is stopped (completely stopped, not just paused) and that the application is free running before taking current readings.



RTOS Configuration and Usage Details


ARM Cortex-M4 FreeRTOS port specific configuration

Configuration items specific to this demo are contained in FreeRTOS/Demo/CORTEX_M4_ATSAM4L_Atmel_Studio/src/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 4 priority bits implemented in the SAM4 NVIC. These values are provided because the CMSIS library function NVIC_SetPriority() 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 Atmel SAM4 ARM Cortex-M4 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.

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

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 USART1_Handler() in the file serial.c included in this demo project for another example.

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 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

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

Source/Portable/MemMang/heap_4.c is included in the ARM Cortex-M4 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.


Miscellaneous

Note that vPortEndScheduler() has not been implemented.





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