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[Task Control]

task. h

BaseType_t xTaskDelayUntil( TickType_t *pxPreviousWakeTime, const TickType_t xTimeIncrement );

INCLUDE_xTaskDelayUntil must be defined as 1 for this function to be available. See the RTOS Configuration documentation for more information.

Delay a task until a specified time. This function can be used by periodic tasks to ensure a constant execution frequency.

This function differs from vTaskDelay() in one important aspect: vTaskDelay() will cause a task to block for the specified number of ticks from the time vTaskDelay() is called whereas xTaskDelayUntil() will cause a task to block for the specified number of ticks from the time specified in the pxPreviousWakeTime parameter. It is difficult to use vTaskDelay() by itself to generate a fixed execution frequency, as the time between a task starting to execute and that task calling vTaskDelay() may not be fixed [the task may take a different path through the code between calls, or may get interrupted or preempted a different number of times each time it executes]. xTaskDelayUntil() can be used to generate a constant execution frequency.

Whereas vTaskDelay() specifies a wake time relative to the time at which the function is called, xTaskDelayUntil() specifies the absolute (exact) time at which it wishes to unblock.

The macro pdMS_TO_TICKS() can be used to calculate the number of ticks from a time specified in milliseconds with a resolution of one tick period.

pxPreviousWakeTime  Pointer to a variable that holds the time at which the task was last unblocked. The variable must be initialised with the current time prior to its first use (see the example below). Following this the variable is automatically updated within xTaskDelayUntil().
xTimeIncrement  The cycle time period. The task will be unblocked at time (*pxPreviousWakeTime + xTimeIncrement). Calling xTaskDelayUntil with the same xTimeIncrement parameter value will cause the task to execute with a fixed interval period.
A value which can be used to check whether the task was actually delayed: pdTRUE if the task way delayed and pdFALSE otherwise. A task will not be delayed if the next expected wake time is in the past.

Example usage:

// Perform an action every 10 ticks.
void vTaskFunction( void * pvParameters )
TickType_t xLastWakeTime;
const TickType_t xFrequency = 10;
BaseType_t xWasDelayed;

    // Initialise the xLastWakeTime variable with the current time.
    xLastWakeTime = xTaskGetTickCount ();
    for( ;; )
        // Wait for the next cycle.
        xWasDelayed = xTaskDelayUntil( &xLastWakeTime, xFrequency );

        // Perform action here. xWasDelayed value can be used to determine
        // whether a deadline was missed if the code here took too long.

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