Chapter 2. API Reference
(continued from previous page)
* // key is pressed, an LCD back-light is switched on. If 5 seconds pass
* // without a key being pressed, then the LCD back-light is switched off. In
* // this case, the timer is a one-shot timer, and unlike the example given for
* // the xTimerReset() function, the key press event handler is an interrupt
* // service routine.
*
* // The callback function assigned to the one-shot timer. In this case the
* // parameter is not used.
* void vBacklightTimerCallback( TimerHandle_t pxTimer )
* {
* // The timer expired, therefore 5 seconds must have passed since a key
* // was pressed. Switch off the LCD back-light.
* vSetBacklightState( BACKLIGHT_OFF );
* }
*
* // The key press interrupt service routine.
* void vKeyPressEventInterruptHandler( void )
* {
* BaseType_t xHigherPriorityTaskWoken = pdFALSE;
*
* // Ensure the LCD back-light is on, then restart the timer that is
* // responsible for turning the back-light off after 5 seconds of
* // key inactivity. This is an interrupt service routine so can only
* // call FreeRTOS API functions that end in "FromISR".
* vSetBacklightState( BACKLIGHT_ON );
*
* // xTimerStartFromISR() or xTimerResetFromISR() could be called here
* // as both cause the timer to re-calculate its expiry time.
* // xHigherPriorityTaskWoken was initialised to pdFALSE when it was
* // declared (in this function).
* if( xTimerStartFromISR( xBacklightTimer, &xHigherPriorityTaskWoken ) !=␣
,→pdPASS )
* {
* // The start command was not executed successfully. Take appropriate
* // action here.
* }
*
* // Perform the rest of the key processing 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 start command 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, although the timers expiry time is relative to when xTimerStartFromISR() is actually called.
The timer service/daemon task priority is set by the configTIMER_TASK_PRIORITY configuration
constant.
Parameters
• xTimer: The handle of the timer being started/restarted.
• pxHigherPriorityTaskWoken: The timer service/daemon task spends most of its time in
Espressif Systems 927
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