Changing timer period at runtime in FreeRTOS - Mini Project: Build & Apply

Changing timer period at runtime

📖 Scenario: You are working on a FreeRTOS-based embedded system that uses a software timer to toggle an LED. You want to learn how to change the timer period while the system is running.

🎯 Goal: Create a FreeRTOS software timer with an initial period. Then change the timer period at runtime to a new value. Finally, print a message to confirm the timer period change.

📋 What You'll Learn

Create a FreeRTOS software timer named xTimer with an initial period of 1000 milliseconds.

Create a variable newPeriod with the value 500 milliseconds.

Change the timer period of xTimer to newPeriod at runtime using the correct FreeRTOS API.

Print the message "Timer period changed to 500 ms" after changing the period.

💡 Why This Matters

🌍 Real World

Changing timer periods at runtime is useful in embedded systems where timing requirements can change, such as adjusting sensor read intervals or LED blink rates.

💼 Career

Embedded software engineers often need to manage timers dynamically to optimize system performance and power consumption.

1

Create a FreeRTOS software timer

Create a FreeRTOS software timer called xTimer with the name "Timer", an initial period of pdMS_TO_TICKS(1000), auto-reload set to pdTRUE, no timer ID (NULL), and a dummy callback function called vTimerCallback.

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"

void vTimerCallback(TimerHandle_t xTimer) {
    // Dummy callback
}

int main(void) {
    // Create xTimer with 1000 ms period
    // Your code here
    return 0;
}

Hint

Use xTimerCreate with the parameters: name, period in ticks, auto-reload, timer ID, and callback function.

2

Create a new period variable

Create a variable called newPeriod and set it to pdMS_TO_TICKS(500) to represent 500 milliseconds.

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"
#include 

void vTimerCallback(TimerHandle_t xTimer) {
    // Dummy callback
}

int main(void) {
    TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
    // Create newPeriod variable for 500 ms
    // Your code here
    return 0;
}

Hint

Use TickType_t type and pdMS_TO_TICKS(500) to convert milliseconds to ticks.

3

Change the timer period at runtime

Use the FreeRTOS API xTimerChangePeriod to change the period of xTimer to newPeriod. Use a block time of 0 ticks.

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"
#include 

void vTimerCallback(TimerHandle_t xTimer) {
    // Dummy callback
}

int main(void) {
    TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
    TickType_t newPeriod = pdMS_TO_TICKS(500);
    // Change timer period to newPeriod
    // Your code here
    return 0;
}

Hint

Use xTimerChangePeriod with parameters: timer handle, new period, and block time.

4

Print confirmation message

Add a printf statement to print the message "Timer period changed to 500 ms" after changing the timer period.

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"
#include 

void vTimerCallback(TimerHandle_t xTimer) {
    // Dummy callback
}

int main(void) {
    TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
    TickType_t newPeriod = pdMS_TO_TICKS(500);
    xTimerChangePeriod(xTimer, newPeriod, 0);
    // Print confirmation message
    // Your code here
    return 0;
}

Hint

Use printf to display the message exactly as shown.

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