Quality RTOS & Embedded Software

 Real time embedded FreeRTOS RSS feed 
Quick Start Supported MCUs PDF Books Trace Tools Ecosystem


Special use of vTaskDelayUntil()

Posted by Mathias Zenger on November 30, 2009

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?


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.

[ Back to the top ]    [ About FreeRTOS ]    [ Sitemap ]    [ ]

Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.

Latest News

Meet us at Embedded World. Hall 3A-525.

Hear from Richard Barry at Embedded World. Feb 28, 16:00, Hall 4-428.

Video: Watch James Gosling & Richard Barry at re:Invent, Las Vegas 2017.

FreeRTOS kernel V10.0.1 is available for immediate download. Now MIT licensed.

FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

IAR Partner

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers

STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0

Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers

OpenRTOS and SafeRTOS