Changing timer period at runtime in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of changing FreeRTOS timer period with xTimerChangePeriod

Consider a FreeRTOS software timer created with a 1000 ms period. The timer callback increments a counter. After starting the timer, the period is changed to 500 ms using xTimerChangePeriod. What will be the value of the counter after approximately 1500 ms?

FreeRTOS

static int counter = 0;

void vTimerCallback(TimerHandle_t xTimer) {
    counter++;
}

void main_task(void *params) {
    TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
    xTimerStart(xTimer, 0);
    vTaskDelay(pdMS_TO_TICKS(700));
    xTimerChangePeriod(xTimer, pdMS_TO_TICKS(500), 0);
    vTaskDelay(pdMS_TO_TICKS(800));
    // Check counter value here
}

A 1

B 0

C 3

D 2

💡 Hint

Think about how many timer callbacks happen before and after the period change.

The timer first fires after 1000 ms, incrementing the counter to 1. Then the period changes to 500 ms. After 500 ms more (total 1500 ms), the timer fires again, incrementing the counter to 2.

🧠 Conceptual

intermediate

Effect of xTimerChangePeriod on a stopped FreeRTOS timer

If a FreeRTOS software timer is created but not started, what happens when xTimerChangePeriod is called on it?

A The timer period changes but the timer remains stopped

B The timer period changes and the timer starts automatically

C The function returns an error and the period does not change

D The timer is deleted automatically

💡 Hint

Consider if changing the period also starts the timer.

xTimerChangePeriod changes the timer period but does not start the timer if it is stopped. The timer remains stopped until xTimerStart is called.

🔧 Debug

advanced

Why does changing timer period not take effect immediately?

Given this code snippet, the timer period change does not seem to take effect immediately. What is the most likely cause?

TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
xTimerStart(xTimer, 0);
vTaskDelay(pdMS_TO_TICKS(500));
xTimerChangePeriod(xTimer, pdMS_TO_TICKS(2000), 0);

A xTimerChangePeriod requires the timer to be stopped first

B The timer period change only takes effect after the current period expires

C The timer callback function is blocking and prevents period change

D The timer handle is invalid and causes the change to fail silently

💡 Hint

Think about how FreeRTOS timers handle period changes during an active timer cycle.

In FreeRTOS, changing the timer period with xTimerChangePeriod schedules the new period to take effect after the current timer period completes. It does not reset the timer immediately.

📝 Syntax

advanced

Identify the syntax error in changing FreeRTOS timer period

Which option contains a syntax error when trying to change a FreeRTOS timer period?

A xTimerChangePeriod(xTimer pdMS_TO_TICKS(500), 0);

B xTimerChangePeriod(xTimer, pdMS_TO_TICKS(500));

C xTimerChangePeriod(xTimer, pdMS_TO_TICKS(500), 0);

D xTimerChangePeriod(xTimer, 500, 0);

💡 Hint

Check the commas separating function arguments.

Option A is missing a comma between xTimer and pdMS_TO_TICKS(500), causing a syntax error.

🚀 Application

expert

Calculate timer callback count after multiple period changes

A FreeRTOS timer is created with a 1000 ms period and started at time 0. At 700 ms, the period is changed to 400 ms. At 1300 ms, the period is changed again to 600 ms. Assuming the timer callback increments a counter each time it fires, what is the counter value at 2500 ms?

FreeRTOS

static int counter = 0;

void vTimerCallback(TimerHandle_t xTimer) {
    counter++;
}

void main_task(void *params) {
    TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
    xTimerStart(xTimer, 0);
    vTaskDelay(pdMS_TO_TICKS(700));
    xTimerChangePeriod(xTimer, pdMS_TO_TICKS(400), 0);
    vTaskDelay(pdMS_TO_TICKS(600));
    xTimerChangePeriod(xTimer, pdMS_TO_TICKS(600), 0);
    vTaskDelay(pdMS_TO_TICKS(1200));
    // Check counter value here
}

A 4

B 6

C 3

D 5

💡 Hint

Calculate each timer firing interval carefully after each period change.

Timer firing timeline (assuming period changes take effect after the current period expires):

• t=1000 ms: first firing, counter=1. Period now 400 ms (change at 700 ms effective here), next at 1400 ms.
• t=1300 ms: period changed to 600 ms, effective after next firing (1400 ms).
• t=1400 ms: firing, counter=2. Period now 600 ms, next at 2000 ms.
• t=2000 ms: firing, counter=3. Next at 2600 ms (>2500 ms).

At 2500 ms, counter=3.

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