Starting and stopping timers in FreeRTOS - Step-by-Step Execution

Concept Flow - Starting and stopping timers

Create Timer

↓

Start Timer

↓

Timer Running

↓

Stop Timer

↓

Timer Stopped

↓

Delete Timer (optional)

This flow shows how a timer is created, started, runs, then stopped and optionally deleted in FreeRTOS.

Execution Sample

FreeRTOS

TimerHandle_t xTimer = xTimerCreate("MyTimer", pdMS_TO_TICKS(1000), pdFALSE, 0, vTimerCallback);
xTimerStart(xTimer, 0);
// Timer runs and calls callback after 1000ms
xTimerStop(xTimer, 0);

This code creates a one-shot timer that runs for 1000 milliseconds, starts it, and then stops it.

Execution Table

Step	Function Call	Timer State	Action	Callback Called
1	xTimerCreate	Created but not running	Timer object created	No
2	xTimerStart	Running	Timer started counting down 1000ms	No
3	Timer counts down	Running	Waiting for timer to expire	No
4	Timer expires	Stopped	Callback function called	Yes
5	xTimerStop	Stopped	Timer stopped	No

💡 Timer stopped or expired, no further callbacks until restarted

Variable Tracker

Variable	Start	After Create	After Start	After Expire	After Stop
xTimer	NULL	Valid Timer Handle	Running	Stopped	Stopped

Key Moments - 3 Insights

Why doesn't the callback run immediately after starting the timer?

Because the timer counts down the specified time (1000ms here) before calling the callback, as shown in execution_table step 3.

What happens if you stop the timer before it expires?

The timer stops counting and the callback is not called.

Can you start a timer again after stopping it?

Yes, you can call xTimerStart again to restart the timer, resetting its countdown.

Visual Quiz - 3 Questions

Look at the execution_table, what is the timer state right after xTimerStart is called?

A Stopped

B Created but not running

C Running

D Deleted

💡 Hint

Check execution_table row 2 under 'Timer State'

At which step does the callback function get called?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Callback Called' column in execution_table

If you stop the timer before it expires, what happens to the callback?

A It is never called

B It is called immediately

C It is called after a delay

D It is called twice

💡 Hint

See execution_table step 5 where timer is stopped and callback is not called

Concept Snapshot

FreeRTOS timers are created with xTimerCreate.
Start a timer with xTimerStart to begin countdown.
Timer calls callback when time expires.
Stop timer anytime with xTimerStop to prevent callback.
Timers can be restarted after stopping.
Deleting timers frees resources but is optional.

Full Transcript

In FreeRTOS, you first create a timer using xTimerCreate, which sets up the timer but does not start it. Then, you start the timer with xTimerStart, which begins the countdown. The timer runs until it expires, at which point it calls the callback function you provided. You can stop the timer anytime before it expires using xTimerStop, which prevents the callback from running. After stopping, you can start the timer again if needed. Optionally, you can delete the timer to free resources. This process helps manage timed events in your embedded system.

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