FreeRTOS Tick Code
[RTOS Implementation Building Blocks]

The actual source code used by the FreeRTOS AVR port is slightly different to the examples shown on the previous pages. vPortYieldFromTick() is itself implemented as a ‘naked’ function, and the context is saved and restored within vPortYieldFromTick(). It is done this way due to the implementation of non-preemptive context switches (where a task blocks itself) – which are not described here.

The FreeRTOS implementation of the RTOS tick is therefore (see the comments in the source code snippets for further details):

void SIG_OUTPUT_COMPARE1A( void ) __attribute__ ( ( signal, naked ) ); void vPortYieldFromTick( void ) __attribute__ ( ( naked ) );


/* Interrupt service routine for the RTOS tick. */ void SIG_OUTPUT_COMPARE1A( void ) { /* Call the tick function. */ vPortYieldFromTick();

/* Return from the interrupt. If a context switch has occurred this will return to a different task. */ asm volatile ( "reti" ); } /*--------------------------------------------------*/

void vPortYieldFromTick( void ) { /* This is a naked function so the context is saved. */ portSAVE_CONTEXT();

/* Increment the tick count and check to see if the new tick value has caused a delay period to expire. This function call can cause a task to become ready to run. */ vTaskIncrementTick();

/* See if a context switch is required. Switch to the context of a task made ready to run by vTaskIncrementTick() if it has a priority higher than the interrupted task. */ vTaskSwitchContext();

/* Restore the context. If a context switch has occurred this will restore the context of the task being resumed. */ portRESTORE_CONTEXT();

/* Return from this naked function. */ asm volatile ( "ret" ); } /*--------------------------------------------------*/

Next: RTOS Implementation – The AVR Context

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