Starting and stopping timers in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function is used to start a FreeRTOS software timer?

The function xTimerStart() is used to start a FreeRTOS software timer. It takes the timer handle and a block time as parameters.

beginner

How do you stop a running FreeRTOS timer?

You stop a running FreeRTOS timer by calling xTimerStop() with the timer handle and a block time.

intermediate

What does the block time parameter in xTimerStart() and xTimerStop() control?

The block time parameter controls how long the calling task will wait if the command queue is full before the start or stop command can be sent.

intermediate

Can a timer be restarted if it is already running in FreeRTOS?

No, calling xTimerStart() on a running timer has no effect. Calling xTimerReset() will reset its timer period and restart the countdown.

advanced

What is the difference between xTimerStop() and xTimerDelete()?

xTimerStop() stops the timer but keeps it available for restarting. xTimerDelete() removes the timer completely and frees its resources.

Which function starts a FreeRTOS software timer?

A xTimerStart()

B xTimerStop()

C xTimerDelete()

D vTaskDelay()

xTimerStart() is used to start a software timer in FreeRTOS.

What happens if you call xTimerStart() on a timer that is already running?

A The timer stops permanently

B The timer is restarted and its countdown resets

C An error is returned

D Nothing happens

Calling xTimerStart() on a running timer has no effect (use xTimerReset() to reset it).

Which parameter controls how long a task waits when starting or stopping a timer if the command queue is full?

A Timer period

B Timer ID

C Block time

D Priority

The block time parameter controls the wait time if the command queue is full.

Which function completely removes a FreeRTOS timer and frees its resources?

A xTimerReset()

B xTimerDelete()

C xTimerStop()

D xTimerStart()

xTimerDelete() removes the timer and frees its resources.

What does xTimerStop() do to a timer?

A Stops the timer but keeps it available

B Changes the timer period

C Restarts the timer

D Deletes the timer

xTimerStop() stops the timer but keeps it available for restarting.

Explain how to start and stop a software timer in FreeRTOS, including the role of the block time parameter.

Think about the functions and what the block time means.

Describe the difference between stopping a timer and deleting a timer in FreeRTOS.

One keeps the timer for reuse, the other removes it completely.

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