Changing timer period at runtime in FreeRTOS

Introduction

Sometimes you want a timer to run faster or slower while your program is running. Changing the timer period at runtime lets you do that easily.

You want to speed up or slow down a repeating task based on user input.

You need to adjust how often a sensor is checked depending on conditions.

You want to save power by reducing timer frequency when the system is idle.

You want to change the blinking speed of an LED dynamically.

You want to test different timer intervals without restarting the system.

Syntax

FreeRTOS

BaseType_t xTimerChangePeriod(TimerHandle_t xTimer, TickType_t xNewPeriod, TickType_t xTicksToWait);

xTimer is the timer you want to change.

xNewPeriod is the new timer period in ticks.

Examples

Change the timer period to 500 milliseconds without waiting.

FreeRTOS

xTimerChangePeriod(myTimer, pdMS_TO_TICKS(500), 0);

Change the timer period to 1 second and wait indefinitely if needed.

FreeRTOS

xTimerChangePeriod(myTimer, pdMS_TO_TICKS(1000), portMAX_DELAY);

Sample Program

This program creates a timer that calls a function every 500 ms. After the timer callback runs 3 times, it changes the timer period to 1000 ms (1 second). The timer keeps running with the new period.

FreeRTOS

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

TimerHandle_t myTimer;

void vTimerCallback(TimerHandle_t xTimer) {
    static int count = 0;
    printf("Timer callback called %d times\n", ++count);
    if (count == 3) {
        // Change timer period to 1 second after 3 calls
        xTimerChangePeriod(xTimer, pdMS_TO_TICKS(1000), 0);
        printf("Timer period changed to 1000 ms\n");
    }
}

int main(void) {
    myTimer = xTimerCreate("MyTimer", pdMS_TO_TICKS(500), pdTRUE, NULL, vTimerCallback);
    if (myTimer == NULL) {
        printf("Timer creation failed\n");
        return 1;
    }

    if (xTimerStart(myTimer, 0) != pdPASS) {
        printf("Timer start failed\n");
        return 1;
    }

    // Start the scheduler
    vTaskStartScheduler();

    // Should never reach here
    return 0;
}

Important Notes

Changing the timer period while the timer is running is safe and common.

Use pdMS_TO_TICKS() to convert milliseconds to ticks for portability.

The xTicksToWait parameter controls how long the function waits if the command queue is full; usually 0 or portMAX_DELAY is used.

Summary

You can change a FreeRTOS timer's period while it is running.

Use xTimerChangePeriod() with the timer handle and new period.

This helps adjust timing dynamically without stopping the timer.

Practice

1.

What function is used in FreeRTOS to change the period of a timer while it is running?

easy

A. xTimerStop()

B. xTimerStart()

C. xTimerReset()

D. xTimerChangePeriod()

Solution

1. Step 1: Understand timer period change in FreeRTOS

   FreeRTOS allows changing a timer's period without stopping it.

2. Step 2: Identify the correct API function

   The function xTimerChangePeriod() is designed to change the timer period at runtime.

3. Final Answer:

   xTimerChangePeriod() -> Option D

4. Quick Check:

   Change timer period = xTimerChangePeriod() [OK]

Hint: Use xTimerChangePeriod() to update timer period on the fly [OK]

Common Mistakes:

• Confusing xTimerStart() with changing period
• Trying to stop and restart timer instead
• Using xTimerReset() which resets but doesn't change period

2.

Which of the following is the correct syntax to change a FreeRTOS timer period to 100 ticks?

?

easy

A. xTimerChangePeriod(timerHandle, 0, 100);

B. xTimerChangePeriod(100, timerHandle, 0);

C. xTimerChangePeriod(timerHandle, 100, 0);

D. xTimerChangePeriod(0, 100, timerHandle);

Solution

1. Step 1: Recall function parameter order

   The function xTimerChangePeriod() takes parameters as (TimerHandle_t xTimer, TickType_t xNewPeriod, TickType_t xTicksToWait).

2. Step 2: Match parameters to example

   To set period to 100 ticks, pass timerHandle, 100, and 0 for no wait.

3. Final Answer:

   xTimerChangePeriod(timerHandle, 100, 0); -> Option C

4. Quick Check:

   Correct parameter order = xTimerChangePeriod(timerHandle, 100, 0); [OK]

Hint: Remember: timer handle first, then new period, then wait ticks [OK]

Common Mistakes:

• Swapping timer handle and period parameters
• Passing wait time before period
• Using wrong parameter types or order

3.

Given the code snippet below, what will be the output if the timer callback prints the timer period each time it runs?

TimerHandle_t timer = xTimerCreate("MyTimer", pdMS_TO_TICKS(500), pdTRUE, NULL, TimerCallback);
xTimerStart(timer, 0);
xTimerChangePeriod(timer, pdMS_TO_TICKS(1000), 0);
// TimerCallback prints current period in ms

medium

A. The callback prints 500 ms once, then 1000 ms repeatedly

B. The callback prints 1000 ms repeatedly from start

C. The callback prints 500 ms repeatedly without change

D. The callback never runs due to period change

Solution

1. Step 1: Analyze timer creation and start

   The timer is created with 500 ms period and started, scheduling first callback in 500 ms.

2. Step 2: Effect of period change

   xTimerChangePeriod() removes the pending expiration, updates the period to 1000 ms, and schedules the next callback in 1000 ms from the change time. Thus, all callbacks print 1000 ms repeatedly.

3. Final Answer:

   The callback prints 1000 ms repeatedly from start -> Option B

4. Quick Check:

   Period change cancels pending expiry = prints 1000 ms repeatedly [OK]

Hint: xTimerChangePeriod cancels pending expiry and starts new full period [OK]

Common Mistakes:

• Thinking first callback uses old 500 ms period
• Assuming period change preserves pending expiration time
• Believing the timer prints 500 ms first then switches

4.

Identify the error in the following code that attempts to change a timer period at runtime:

TimerHandle_t timer = xTimerCreate("Timer", pdMS_TO_TICKS(200), pdTRUE, NULL, TimerCallback);
xTimerChangePeriod(200, timer, 0);

medium

A. Parameters to xTimerChangePeriod are in wrong order

B. Timer was not started before changing period

C. Timer callback function is missing

D. pdMS_TO_TICKS() cannot be used in timer period

Solution

1. Step 1: Check function parameter order

   xTimerChangePeriod() expects (TimerHandle_t, TickType_t, TickType_t), but code passes (TickType_t, TimerHandle_t, TickType_t).

2. Step 2: Confirm other code parts

   Timer is created correctly and callback is provided; pdMS_TO_TICKS() is valid for period conversion.

3. Final Answer:

   Parameters to xTimerChangePeriod are in wrong order -> Option A

4. Quick Check:

   Correct parameter order is timer handle first [OK]

Hint: Timer handle always comes first in xTimerChangePeriod() [OK]

Common Mistakes:

• Swapping timer handle and period parameters
• Not starting timer before changing period (not required)
• Misunderstanding pdMS_TO_TICKS usage

5.

You have a FreeRTOS timer running with a 200 ms period. You want to double the period each time the timer callback runs, up to a maximum of 1600 ms. Which code snippet correctly changes the timer period at runtime inside the callback?

void TimerCallback(TimerHandle_t xTimer) {
    TickType_t currentPeriod = xTimerGetPeriod(xTimer);
    TickType_t newPeriod = currentPeriod * 2;
    if (newPeriod <= pdMS_TO_TICKS(1600)) {
        ?
    }
}

hard

A. xTimerChangePeriod(xTimer, newPeriod, 0);

B. xTimerStart(xTimer, 0);

C. xTimerStop(xTimer, 0); xTimerChangePeriod(xTimer, newPeriod, 0);

D. xTimerReset(xTimer, 0); xTimerChangePeriod(xTimer, newPeriod, 0);

Solution

1. Step 1: Understand timer period update inside callback

   Inside the callback, you can call xTimerChangePeriod() to update the timer period dynamically.

2. Step 2: Choose correct function usage

   Calling xTimerChangePeriod(xTimer, newPeriod, 0); changes the period without stopping or restarting the timer.

3. Final Answer:

   xTimerChangePeriod(xTimer, newPeriod, 0); -> Option A

4. Quick Check:

   Change period directly inside callback = xTimerChangePeriod(xTimer, newPeriod, 0); [OK]

Hint: Use xTimerChangePeriod inside callback to update period safely [OK]

Common Mistakes:

• Stopping timer before changing period unnecessarily
• Starting timer again inside callback
• Resetting timer which restarts count instead of changing period