The vTaskDelayUntil() API Function. FreeRTOS Real Time Kernel

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Not Running

(super state)

Suspended vTaskSuspend() called vTaskResume() called vTaskSuspend() called

Ready Running vTaskSuspend() called

Event

Blocking API function called

Blocked

Figure 18. Bold lines indicate the state transitions performed

by the tasks in Example 4

The vTaskDelayUntil() API Function

vTaskDelayUntil() is similar to vTaskDelay(). As just demonstrated, the vTaskDelay() parameter specifies the number of tick interrupts that should occur between a task calling vTaskDelay(), and the same task once again transitioning out of the Blocked state. The length of time the task remains in the blocked state is specified by the vTaskDelay() parameter, but the time at which the task leaves the blocked state is relative to the time at which vTaskDelay() was called.

The parameters to vTaskDelayUntil() specify, instead, the exact tick count value at which the calling task should be moved from the Blocked state into the Ready state. vTaskDelayUntil() is the API function that should be used when a fixed execution period is required (where you want your task to execute periodically with a fixed frequency), as the time at which the calling task is unblocked is absolute, rather than relative to when the function was called (as is the case with vTaskDelay()).

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