|
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
Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.
|
