Atmel SAMD20 ARM Cortex-M0+ Demo
FreeRTOS and FreeRTOS-Plus-CLI Built Using Atmel Studio and GCC
SAMD20 Xplained PRO
This page describes a demo project that uses FreeRTOS and
on the SAMD20 Cortex-M0+ microcontroller from Atmel.
The project uses the FreeRTOS ARM Cortex-M0 GCC port, builds with the free
Atmel Studio IDE (which uses the Visual Studio framework
and includes a kernel aware FreeRTOS plug-in), and targets the
very low cost SAMD20 Xplained Pro
The command line interface character input and output uses drivers provided by
Atmel in their Atmel Software Framework (ASF).
A #define is used to switch the build between a simple blinky style application,
and a comprehensive test and demo application that incorporates the FreeRTOS-Plus-CLI
The Atmel Studio IDE with Kernel Aware FreeRTOS Plug-in
IMPORTANT! Notes on using the SAMD20 ARM Cortex-M0+ Demo
Please read all the following points before using this RTOS port.
See also the FAQ My application does
not run, what could be wrong?
- Source Code Organisation
- The Demo Application
- RTOS Configuration and Usage Details
Source Code Organisation
The FreeRTOS download contains the source code for all the FreeRTOS ports, so
contains many more files than are needed by the SAMD20 demo.
See the Source Code Organization
section for a description of the directory structure.
The Atmel Studio solution file is called RTOSDemo.atsln, and is located in the
Building and Running the ARM Cortex-M0+ RTOS Application
Open FreeRTOS/Demo/CORTEX_M0+_Atmel_SAMD20_Xplained/RTOSDemo.atsln in the
Atmel Studio IDE.
Locate the mainCREATE_SIMPLE_BLINKY_DEMO_ONLY definition at the top of
main.c. Set mainCREATE_SIMPLE_BLINKY_DEMO_ONLY to 1 to create the simple
blinky demo, or 0 to create the comprehensive demo that also includes
the command line interpreter.
Select "Rebuild RTOSDemo" from the Atmel Studio "Build" menu (or press F7) to build
the demo project.
Connect a USB cable between the USB port on the SAMD20 Xplained Pro board
and the host computer.
Select "Start Debugging and Break" from the Atmel Studio "Debug"
menu to program the microcontroller flash memory and start a debug
Demo Application Functionality
Functionality with mainCREATE_SIMPLE_BLINKY_DEMO_ONLY set to 1
Building with mainCREATE_SIMPLE_BLINKY_DEMO_ONLY set to 1 results in main() calling
main_blinky(). The main_blinky() demo is described below.
The main_blinky() Function:
main_blinky() creates one queue, and two tasks. It then starts the
The Queue Send Task:
The queue send task is implemented by prvQueueSendTask() in main_blinky.c.
prvQueueSendTask() repeatedly blocks for 200 milliseconds before sending
the value 100 to the queue that was created in main_blinky().
The Queue Receive Task:
The queue receive task is implemented by prvQueueReceiveTask()
prvQueueReceiveTask() repeatedly blocks on attempts to read from the queue
that was created in main_blinky(), toggling the LED each time data is
received. The queue send task sends data to the queue every 200 milliseconds,
so the LED will toggle every 200 milliseconds.
Functionality with mainCREATE_SIMPLE_BLINKY_DEMO_ONLY set to 0
Building with mainCREATE_SIMPLE_BLINKY_DEMO_ONLY set to 0 results in main() calling
main_full(). The main_full() demo is described below.
The main_full() Function:
main_full() creates a set of standard demo tasks, some application specific
test tasks, a task that manages FreeRTOS-Plus-CLI, and a timer. It then starts the
Click to enlarge
The SAMD20 Xplained Pro board connects to a host computer by a single
USB cable. The USB connection provides both a debugger interface and
a virtual COM port connection. The virtual COM port is used to provide
the character input and output required by the command line interface
(CLI). The CLI can therefore be accessed from any serial terminal program,
such as Hyper Terminal, or as shown in the image on the right, Tera Term.
The image shows COM 20 being used, but it is likely that the virtual
COM port will enumerate to a different port number on your host computer.
As always with FreeRTOS-Plus-CLI, the 'help' command will display a list of
registered (and therefore available) commands. The image was captured
after executing the 'run-time-stats' command, which generates a table
showing the amount of CPU time that has been consumed by each RTOS task.
The "Reg Test" Tasks:
These fill the registers with known values, then check
that each register maintains its expected value for the lifetime of the
task. Each task uses a different set of values. A register
containing an unexpected value is indicative of an error in the context
The "Check" software Timer:
The check software timer period is initially set to three
seconds. Its callback function checks that all the standard demo tasks, and
the register check tasks, are not only still executing, but are executing
without reporting any errors. If the check timer callback discovers that a
task has either stalled, or reported an error, then it changes the period of
the check timer from the initial three seconds, to just 200ms. The callback
function also toggles the LED to give a visual
indication of the system status: If the LED is toggling every three seconds
then no issues have been discovered. If the LED is toggling every 200ms, then
a problem has been discovered in at least one task.
Interrupt service routines
Interrupt service routines that cause a context switch have
no special requirements.
The macro portEND_SWITCHING_ISR() can be used to request a context switch from within an ISR.
Note that portEND_SWITCHING_ISR() will leave interrupts enabled.
A dummy interrupt handler
called Dummy_IRQHandler() is provided at the end of main.c as a reference implementation.
Dummy_IRQHandler() is also replicated below.
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. Only FreeRTOS API functions
that end in "FromISR" can be called from an ISR! */
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 );
Note that the following lines are included in FreeRTOSConfig.h to map the FreeRTOS
interrupt handler function names onto the CMSIS interrupt handler function names.
This allows the linker scripts provided by the compiler tool vendors to be used
#define vPortSVCHandler SVC_Handler
#define xPortPendSVHandler PendSV_Handler
#define xPortSysTickHandler SysTick_Handler
RTOS port specific configuration
Configuration items specific to these demos are contained in FreeRTOS/Demo/CORTEX_M0+_Atmel_SAMD20_Xplained/RTOSDemo/src/config/FreeRTOSConfig.h. The
constants defined in FreeRTOSConfig.h can be edited to meet the needs of your application. In particular -
Each port #defines 'BaseType_t' to equal the most efficient data type for that
processor. All ARM Cortex-M0+ ports define BaseType_t to be of type long.
Note that vPortEndScheduler() has not been implemented.
Source/Portable/MemMang/heap_4.c is included in the ARM Cortex-M0+ 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
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