Changing timer period at runtime in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function is used to change the period of a FreeRTOS software timer at runtime?

The function xTimerChangePeriod() is used to change the timer period while the timer is active or inactive.

beginner

What parameters does xTimerChangePeriod() require?

It requires the timer handle, the new timer period in ticks, and the block time to wait for the command to be sent to the timer command queue.

intermediate

What happens if you call xTimerChangePeriod() on a timer that is not active?

The timer period is updated, but the timer will not start running until xTimerStart() or xTimerStartFromISR() is called.

beginner

Why might you want to change a timer period at runtime in FreeRTOS?

To adjust the timing of periodic tasks dynamically based on system conditions, such as changing sensor read intervals or adapting to workload.

intermediate

What is the difference between xTimerChangePeriod() and xTimerReset()?

xTimerChangePeriod() changes the timer period and optionally restarts the timer, while xTimerReset() restarts the timer with the current period without changing it.

Which FreeRTOS function changes the timer period at runtime?

A xTimerStop()

B xTimerChangePeriod()

C xTimerReset()

D xTimerStart()

xTimerChangePeriod() is the correct function to change the timer period dynamically.

What must you provide to xTimerChangePeriod() besides the timer handle?

A Timer name

B Callback function

C Priority level

D New period in ticks and block time

You provide the new timer period in ticks and the block time to wait for the command queue.

If a timer is inactive, what happens when you call xTimerChangePeriod()?

A Timer is deleted

B Timer starts immediately

C Period changes but timer stays stopped

D Function returns an error

The timer period updates but the timer remains stopped until started explicitly.

Which of these is NOT a reason to change a timer period at runtime?

A Fix a bug in the timer code

B Adjust task timing dynamically

C Adapt to workload changes

D Change sensor read intervals

Changing the timer period is for timing adjustments, not for fixing bugs.

How does xTimerChangePeriod() differ from xTimerReset()?

A Changes period vs restarts with same period

B Starts vs stops timer

C Deletes vs creates timer

D Changes priority vs changes callback

xTimerChangePeriod() changes the timer period; xTimerReset() restarts the timer with the current period.

Explain how to change a FreeRTOS timer period at runtime and what parameters you need.

Think about the function name and what it needs to do its job.

Describe a practical scenario where changing a timer period at runtime is useful in an embedded system.

Consider how system needs might change during operation.

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