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Special use of vTaskDelayUntil()

Posted by Mathias Zenger on November 30, 2009
Hi,

Normally vTaskDelayUntil() is used where you want to execute your task periodically with a fixed frequency. I want to run my task with a frequency of 50Hz as long as two global values are different. Then the task is blocking on a queue waiting for a command.

My problem: When executing my code like shown below I sometimes get a strange behaviour. The inner while loop which shall be executed with 50Hz repeats much too fast without any delay.

My question: When using vTaskDelayUntil() in this context the single initialization prior to the first while loop does not seem to be sufficient. Must I call xTaskGetTickCount() every time I enter the inner while loop?

Snippet:

static portTASK_FUNCTION(vSampleTask, pvParameters) {
portBASE_TYPE xRes;
xTasksvTypeCommand xCmdIn;
portTickType xLastWakeTime;

// Initialize with current tick count
xLastWakeTime = xTaskGetTickCount();

// Task loop
while (1) {
while (global_a != global_b) {
// Do some periodic action
// ...
vTaskDelayUntil(&xLastWakeTime, 20 / portTICK_RATE_MS);
}
// Block until any command has been written to the task command queue
xRes = xQueueReceive(xCommandQueue, &xCmdIn, portMAX_DELAY);
if (xRes == pdPASS) {
// Process command
// ...
}
}
}

Thank you for your comments! Regards

RE: Special use of vTaskDelayUntil()

Posted by Dave on November 30, 2009
You must re-init xLastWakeTime after you unblock from the queue.


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