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