Chapter 2. API Reference
formed before the interrupt exits.
xTimerChangePeriodFromISR(xTimer, xNewPeriod, pxHigherPriorityTaskWoken)
BaseType_t xTimerChangePeriodFromISR( TimerHandle_t xTimer, TickType_t xNewPeriod, BaseType_t
*pxHigherPriorityTaskWoken );
A version of xTimerChangePeriod() that can be called from an interrupt service routine.
Example usage:
* // This scenario assumes xTimer has already been created and started. When
* // an interrupt occurs, the period of xTimer should be changed to 500ms.
*
* // The interrupt service routine that changes the period of xTimer.
* void vAnExampleInterruptServiceRoutine( void )
* {
* BaseType_t xHigherPriorityTaskWoken = pdFALSE;
*
* // The interrupt has occurred - change the period of xTimer to 500ms.
* // xHigherPriorityTaskWoken was set to pdFALSE where it was defined
* // (within this function). As this is an interrupt service routine, only
* // FreeRTOS API functions that end in "FromISR" can be used.
* if( xTimerChangePeriodFromISR( xTimer, &xHigherPriorityTaskWoken ) !=␣
,→pdPASS )
* {
* // The command to change the timers period was not executed
* // successfully. Take appropriate action here.
* }
*
* // If xHigherPriorityTaskWoken equals pdTRUE, then a context switch
* // should be performed. The syntax required to perform a context switch
* // from inside an ISR varies from port to port, and from compiler to
* // compiler. Inspect the demos for the port you are using to find the
* // actual syntax required.
* if( xHigherPriorityTaskWoken != pdFALSE )
* {
* // Call the interrupt safe yield function here (actual function
* // depends on the FreeRTOS port being used).
* }
* }
*
Return pdFAIL will be returned if the command to change the timers period could not be sent to the timer
command queue. pdPASS will be returned if the command was successfully sent to the timer command
queue. When the command is actually processed will depend on the priority of the timer service/daemon
task relative to other tasks in the system. The timer service/daemon task priority is set by the config-
TIMER_TASK_PRIORITY configuration constant.
Parameters
• xTimer: The handle of the timer that is having its period changed.
• xNewPeriod: The new period for xTimer. Timer periods are specified in tick periods, so the
constant portTICK_PERIOD_MS can be used to convert a time that has been specified in millisec-
onds. For example, if the timer must expire after 100 ticks, then xNewPeriod should be set to
100. Alternatively, if the timer must expire after 500ms, then xNewPeriod can be set to ( 500 /
portTICK_PERIOD_MS ) provided configTICK_RATE_HZ is less than or equal to 1000.
• pxHigherPriorityTaskWoken: The timer service/daemon task spends most of its time in
the Blocked state, waiting for messages to arrive on the timer command queue. Calling xTimer-
ChangePeriodFromISR() writes a message to the timer command queue, so has the potential to
transition the timer service/ daemon task out of the Blocked state. If calling xTimerChangePe-
riodFromISR() causes the timer service/daemon task to leave the Blocked state, and the timer
service/daemon task has a priority equal to or greater than the currently executing task (the task
that was interrupted), then *pxHigherPriorityTaskWoken will get set to pdTRUE internally within
the xTimerChangePeriodFromISR() function. If xTimerChangePeriodFromISR() sets this value to
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