vTaskDelayUntil() for precise timing in FreeRTOS

Introduction

vTaskDelayUntil() helps tasks run at exact time intervals. It keeps timing steady even if the task takes different amounts of time to run.

You want a task to run every 100 milliseconds exactly.

You need to blink an LED at a steady rate without drift.

You want to sample a sensor at fixed time intervals.

You want to avoid timing errors building up over many cycles.

Syntax

FreeRTOS

void vTaskDelayUntil(TickType_t *pxPreviousWakeTime, TickType_t xTimeIncrement);

pxPreviousWakeTime is a pointer to a variable holding the last wake time.

xTimeIncrement is how many ticks to wait between runs.

Examples

This example runs the task every 100 milliseconds precisely.

FreeRTOS

TickType_t xLastWakeTime;
xLastWakeTime = xTaskGetTickCount();

for(;;) {
    // Task code here
    vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(100));
}

Delays the task to run every 50 ticks from the last wake time.

FreeRTOS

TickType_t xLastWakeTime = xTaskGetTickCount();
vTaskDelayUntil(&xLastWakeTime, 50);

Sample Program

This program creates a task that prints the current tick count every 500 milliseconds exactly, using vTaskDelayUntil() for precise timing.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

void vTaskFunction(void *pvParameters) {
    TickType_t xLastWakeTime;
    const TickType_t xFrequency = pdMS_TO_TICKS(500); // 500 ms

    xLastWakeTime = xTaskGetTickCount();

    for(;;) {
        printf("Task running at tick: %lu\n", xTaskGetTickCount());
        vTaskDelayUntil(&xLastWakeTime, xFrequency);
    }
}

int main(void) {
    xTaskCreate(vTaskFunction, "Task", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    for(;;);
    return 0;
}

Important Notes

Always initialize pxPreviousWakeTime with xTaskGetTickCount() before the first call.

vTaskDelayUntil() helps avoid drift that happens with simple delay functions.

If the task runs longer than the delay period, the next wake time will be in the past, so the task runs immediately to catch up.

Summary

vTaskDelayUntil() keeps task timing steady and precise.

Use it when you want tasks to run at fixed intervals without drift.

Initialize the last wake time before the loop and pass it by pointer each time.

Practice

1. What is the main purpose of vTaskDelayUntil() in FreeRTOS?

easy

A. To delay a task until a fixed periodic time, keeping timing precise

B. To delay a task for a random amount of time

C. To suspend a task indefinitely

D. To immediately resume a suspended task

Solution

1. Step 1: Understand the function's purpose

   vTaskDelayUntil() is designed to delay a task until a specific time, maintaining a fixed periodic interval.

2. Step 2: Compare options with function behavior

   Only To delay a task until a fixed periodic time, keeping timing precise describes this precise periodic delay. Other options describe unrelated behaviors.

3. Final Answer:

   To delay a task until a fixed periodic time, keeping timing precise -> Option A

4. Quick Check:

   Precise periodic delay = A [OK]

Hint: vTaskDelayUntil keeps timing steady, not random delays [OK]

Common Mistakes:

• Confusing vTaskDelayUntil with vTaskDelay
• Thinking it suspends tasks indefinitely
• Assuming it resumes tasks immediately

2. Which of the following is the correct way to declare the variable used with vTaskDelayUntil() to track the last wake time?

easy

A. TickType_t xLastWakeTime = 0;

B. int xLastWakeTime = 0;

C. TickType_t xLastWakeTime;

D. void *xLastWakeTime = NULL;

Solution

1. Step 1: Identify the correct type for last wake time

   The FreeRTOS API specifies TickType_t as the type for tick counts and time tracking.

2. Step 2: Initialization rules for vTaskDelayUntil

   The variable must be declared but not necessarily initialized to zero; it is initialized by calling xLastWakeTime = xTaskGetTickCount(); before the loop.

3. Final Answer:

   TickType_t xLastWakeTime; -> Option C

4. Quick Check:

   Use TickType_t without initial zero for last wake time [OK]

Hint: Declare with TickType_t, initialize with xTaskGetTickCount() [OK]

Common Mistakes:

• Using int instead of TickType_t
• Initializing to zero instead of using xTaskGetTickCount()
• Using void pointer type

3. Given the code snippet below, what will be the approximate delay between each "Task running" print?

TickType_t xLastWakeTime;
xLastWakeTime = xTaskGetTickCount();

for (;;) {
    vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(1000));
    printf("Task running\n");
}

medium

A. Approximately 1000 milliseconds, steady without drift

B. Exactly 1000 milliseconds between prints

C. Variable delay, sometimes less than 1000 milliseconds

D. No delay, prints continuously

Solution

1. Step 1: Understand vTaskDelayUntil behavior

   It delays the task until the next fixed interval based on the last wake time, keeping timing steady.

2. Step 2: Analyze the delay duration

   The delay is set to 1000 ms converted to ticks, so the task runs approximately every 1000 ms without accumulating drift.

3. Final Answer:

   Approximately 1000 milliseconds, steady without drift -> Option A

4. Quick Check:

   vTaskDelayUntil = steady periodic delay [OK]

Hint: vTaskDelayUntil keeps intervals steady, not exact but no drift [OK]

Common Mistakes:

• Assuming exact millisecond precision
• Confusing with vTaskDelay which causes drift
• Thinking delay varies randomly

4. Identify the error in the following FreeRTOS task code using vTaskDelayUntil():

void vTaskFunction(void *pvParameters) {
    TickType_t xLastWakeTime = 0;
    for (;;) {
        vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(500));
        // Task code here
    }
}

medium

A. vTaskDelayUntil() should not be used inside a loop

B. TickType_t cannot be initialized to zero

C. pdMS_TO_TICKS() cannot be used as a delay parameter

D. xLastWakeTime should be initialized with xTaskGetTickCount() before the loop

Solution

1. Step 1: Check initialization of xLastWakeTime

   It must be initialized to the current tick count using xTaskGetTickCount() before the loop to set the reference time.

2. Step 2: Verify usage of vTaskDelayUntil

   The function is correctly used inside the loop, and pdMS_TO_TICKS is valid for converting milliseconds to ticks.

3. Final Answer:

   xLastWakeTime should be initialized with xTaskGetTickCount() before the loop -> Option D

4. Quick Check:

   Initialize xLastWakeTime with xTaskGetTickCount() [OK]

Hint: Always init last wake time with xTaskGetTickCount() before loop [OK]

Common Mistakes:

• Not initializing xLastWakeTime before loop
• Misusing vTaskDelayUntil outside loops
• Incorrect delay parameter types

5. You want a FreeRTOS task to run exactly every 200 ms without drift, but your task sometimes takes 50 ms to execute. How should you use vTaskDelayUntil() to achieve this?

hard

A. Use vTaskDelayUntil() with 50 ms delay to match task execution time

B. Initialize xLastWakeTime with xTaskGetTickCount() before the loop, then call vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(200)) inside the loop after task code

C. Initialize xLastWakeTime to zero and call vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(250)) before task code

D. Call vTaskDelay(pdMS_TO_TICKS(200)) after task code to delay fixed 200 ms

Solution

1. Step 1: Understand timing requirements

   The task must run every 200 ms total, including 50 ms execution time, so delay should be 200 ms fixed interval.

2. Step 2: Correct usage of vTaskDelayUntil

   Initialize xLastWakeTime with xTaskGetTickCount() before the loop, then call vTaskDelayUntil() with 200 ms delay after task code to maintain steady timing without drift.

3. Final Answer:

   Initialize xLastWakeTime with xTaskGetTickCount() before the loop, then call vTaskDelayUntil(&xLastWakeTime, pdMS_TO_TICKS(200)) inside the loop after task code -> Option B

4. Quick Check:

   Init last wake time + vTaskDelayUntil with full period = D [OK]

Hint: Init last wake time, delay full period after task code [OK]

Common Mistakes:

• Using vTaskDelay instead of vTaskDelayUntil causing drift
• Not accounting for task execution time in delay
• Initializing xLastWakeTime incorrectly