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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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