Starting and stopping timers in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a FreeRTOS timer named "myTimer".

FreeRTOS

TimerHandle_t myTimer = xTimerCreate("myTimer", pdMS_TO_TICKS(1000), pdFALSE, (void *)0, [1]);

Drag options to blanks, or click blank then click option'

AxTimerStop

BxTimerStart

CvTaskDelay

DvTimerCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using xTimerStart or xTimerStop as the callback function.

Passing a delay function instead of a callback.

The last parameter of xTimerCreate is the callback function that runs when the timer expires. Here, it should be vTimerCallback.

2fill in blank

medium

Complete the code to start the timer "myTimer" immediately.

FreeRTOS

if (xTimerStart(myTimer, [1]) != pdPASS) {
    // Handle error
}

Drag options to blanks, or click blank then click option'

ApdFALSE

BportMAX_DELAY

C0

DpdMS_TO_TICKS(1000)

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdMS_TO_TICKS(1000) which causes a delay.

Using portMAX_DELAY which waits indefinitely.

The second parameter of xTimerStart is the block time to wait for the command to be sent to the timer command queue. Using 0 means do not wait.

3fill in blank

hard

Fix the error in stopping the timer "myTimer" with a block time of 10 ms.

FreeRTOS

if (xTimerStop(myTimer, [1]) != pdPASS) {
    // Handle error
}

Drag options to blanks, or click blank then click option'

ApdMS_TO_TICKS(10)

B0

CpdFALSE

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Passing raw numbers like 10 without conversion.

Using pdFALSE which is not a valid block time.

The block time parameter expects ticks. Using pdMS_TO_TICKS(10) converts 10 milliseconds to ticks correctly.

4fill in blank

hard

Fill both blanks to create and start an auto-reload timer named "autoTimer" with a 500 ms period.

FreeRTOS

TimerHandle_t autoTimer = xTimerCreate("autoTimer", [1], [2], NULL, vAutoTimerCallback);
xTimerStart(autoTimer, 0);

Drag options to blanks, or click blank then click option'

ApdMS_TO_TICKS(500)

BpdFALSE

CpdTRUE

DvTimerCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE for auto-reload instead of pdTRUE.

Passing raw numbers instead of using pdMS_TO_TICKS.

The second parameter is the timer period in ticks, so pdMS_TO_TICKS(500) converts 500 ms to ticks. The third parameter is pdTRUE to make the timer auto-reload.

5fill in blank

hard

Fill all three blanks to create a one-shot timer named "oneShot" with a 2000 ms period, start it, and check for success.

FreeRTOS

TimerHandle_t oneShot = xTimerCreate("oneShot", [1], [2], NULL, vOneShotCallback);
if (xTimerStart(oneShot, [3]) != pdPASS) {
    // Handle error
}

Drag options to blanks, or click blank then click option'

ApdMS_TO_TICKS(2000)

BpdFALSE

C0

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdTRUE for one-shot timers.

Not converting milliseconds to ticks.

Using non-zero block time unnecessarily.

The timer period is 2000 ms converted to ticks with pdMS_TO_TICKS(2000). The timer is one-shot, so pdFALSE is used for auto-reload. The start block time is zero to start immediately.

Practice

1. What does the function xTimerCreate do in FreeRTOS?

easy

A. Creates a timer but does not start it automatically

B. Starts a timer immediately after creation

C. Stops a running timer

D. Deletes a timer permanently

Solution

1. Step 1: Understand the purpose of xTimerCreate

   This function is used to create a software timer in FreeRTOS but it does not start the timer automatically.

2. Step 2: Differentiate from other timer functions

   Starting a timer requires xTimerStart, stopping requires xTimerStop, and deleting requires xTimerDelete.

3. Final Answer:

   Creates a timer but does not start it automatically -> Option A

4. Quick Check:

   xTimerCreate only creates timer [OK]

Hint: Remember: create ≠ start; start needs separate call [OK]

Common Mistakes:

• Thinking xTimerCreate starts the timer automatically
• Confusing xTimerStart with creation
• Assuming timer is running immediately after creation

2. Which of the following is the correct way to start a FreeRTOS timer named xTimer?

easy

A. xTimerDelete(xTimer, 0);

B. xTimerCreate(xTimer, 0);

C. xTimerStop(xTimer, 0);

D. xTimerStart(xTimer, 0);

Solution

1. Step 1: Identify the function to start a timer

   The function xTimerStart is used to start a timer in FreeRTOS.

2. Step 2: Check the syntax

   The correct syntax is xTimerStart(xTimer, 0); where the second parameter is the block time.

3. Final Answer:

   xTimerStart(xTimer, 0); -> Option D

4. Quick Check:

   Start timer = xTimerStart call [OK]

Hint: Start timers with xTimerStart, not create or stop [OK]

Common Mistakes:

• Using xTimerCreate to start a timer
• Confusing stop and delete functions with start
• Missing the block time parameter

3. Given the following code snippet, what will be the output if the timer callback increments a counter each time it runs and the timer is started and stopped immediately?

int counter = 0;
TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, timerCallback);
xTimerStart(xTimer, 0);
xTimerStop(xTimer, 0);
// Assume timerCallback increments counter

medium

A. counter will remain 0

B. counter will be incremented at least once

C. Compilation error due to missing parameters

D. Runtime error due to invalid timer handle

Solution

1. Step 1: Analyze timer start and stop sequence

   The timer is started and then immediately stopped before it can expire and call the callback.

2. Step 2: Understand timer callback execution

   Since the timer period is 1000 ms and it is stopped immediately, the callback does not get a chance to run, so the counter stays 0.

3. Final Answer:

   counter will remain 0 -> Option A

4. Quick Check:

   Immediate stop prevents callback = counter 0 [OK]

Hint: Timer callback runs only after timer expires, not immediately [OK]

Common Mistakes:

• Assuming callback runs immediately on start
• Ignoring timer period delay
• Confusing start and stop timing effects

4. Identify the error in the following code snippet that attempts to stop a timer:

TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(500), pdFALSE, NULL, timerCallback);
xTimerStop(xTimer, 1000);

medium

A. The block time parameter in xTimerStop is too large

B. The timer was never started before stopping

C. The timer handle is NULL

D. The timer period is set incorrectly

Solution

1. Step 1: Check timer lifecycle

   The timer is created but never started before calling xTimerStop.

2. Step 2: Understand stopping a timer

   Stopping a timer that is not running has no effect and may cause unexpected behavior.

3. Final Answer:

   The timer was never started before stopping -> Option B

4. Quick Check:

   Stop requires running timer first [OK]

Hint: Always start timer before stopping it [OK]

Common Mistakes:

• Stopping timer without starting it
• Assuming block time causes error
• Misunderstanding timer handle validity

5. You want to create a periodic timer in FreeRTOS that starts automatically and stops after 5 executions. Which approach correctly achieves this behavior?

hard

A. Create the timer with pdTRUE, start it automatically by xTimerCreate, and delete after 5 executions

B. Create the timer with pdFALSE, start it manually, and stop it after 5 seconds

C. Create the timer with pdTRUE for auto-reload, start it manually, and in the callback count executions and stop after 5

D. Create the timer with pdFALSE, start it automatically, and restart it 5 times

Solution

1. Step 1: Understand timer creation parameters

   Setting pdTRUE makes the timer periodic (auto-reload), but it does not start automatically; you must call xTimerStart.

2. Step 2: Implement stopping after 5 executions

   Use a counter inside the callback function to track executions and call xTimerStop after the 5th execution.

3. Final Answer:

   Create the timer with pdTRUE for auto-reload, start it manually, and in the callback count executions and stop after 5 -> Option C

4. Quick Check:

   Periodic timer + manual start + callback stop after 5 [OK]

Hint: Use callback counter to stop periodic timer after N runs [OK]

Common Mistakes:

• Assuming xTimerCreate starts timer automatically
• Using one-shot timer for periodic behavior
• Not counting executions in callback