void vTaskDelayUntil( TickType_t *pxPreviousWakeTime,
const TickType_t xTimeIncrement );
INCLUDE_vTaskDelayUntil 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() specifies a time at
which the task wishes to unblock relative to the time at which vTaskDelay() is called, whereas
vTaskDelayUntil() specifies an absolute time at which the task wishes to unblock.
cause a task to block for the specified number of ticks from the time vTaskDelay() is
called. It is therefore difficult to use vTaskDelay() by itself to generate a fixed
execution frequency as the time between a task unblocking following a call to vTaskDelay() and that task
next 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].
Whereas vTaskDelay() specifies a wake time relative to the time at which the function
is called, vTaskDelayUntil() specifies the absolute (exact) time at which it wishes to
It should be noted that vTaskDelayUntil() will return immediately (without blocking) if
it is used to specify a wake time that is already in the past. Therefore a task using
vTaskDelayUntil() to execute periodically will have to re-calculate its required wake time
if the periodic execution is halted for any reason (for example, the task is temporarily
placed into the Suspended state) causing the task to miss one or more periodic
executions. This can be detected by checking the variable passed by reference as the
pxPreviousWakeTime parameter against the current tick count. This is however not necessary
under most usage scenarios.
The constant portTICK_PERIOD_MS can be used to calculate real time from the tick
rate - with the resolution of one tick period.
This function must not be called while the RTOS scheduler has been suspended by a call to vTaskSuspendAll().
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 vTaskDelayUntil().
The cycle time period. The task will be unblocked at
time (*pxPreviousWakeTime + xTimeIncrement). Calling vTaskDelayUntil with the
same xTimeIncrement parameter value will cause the task to execute with
a fixed interval period. |
// 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.
vTaskDelayUntil( &xLastWakeTime, xFrequency );
// Perform action here.
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