One-shot vs auto-reload timers in FreeRTOS - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a one-shot timer in FreeRTOS.

FreeRTOS

TimerHandle_t timer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), [1], NULL, timerCallback);

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

ApdFALSE

BpdTRUE

CpdONCE

DpdAUTO

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdTRUE creates an auto-reload timer instead of one-shot.

Use pdFALSE to create a one-shot timer that runs only once.

2fill in blank

medium

Complete the code to create an auto-reload timer in FreeRTOS.

FreeRTOS

TimerHandle_t timer = xTimerCreate("Timer", pdMS_TO_TICKS(500), [1], NULL, timerCallback);

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

ApdFALSE

BpdTRUE

CpdONCE

DpdSTOP

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE creates a one-shot timer instead of auto-reload.

Use pdTRUE to create an auto-reload timer that restarts automatically.

3fill in blank

hard

Fix the error in the timer start code to start the timer correctly.

FreeRTOS

BaseType_t started = xTimerStart(timer, [1]);

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

A0

BportMAX_DELAY

CpdFALSE

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdTRUE or pdFALSE as block time causes errors.

The block time parameter should be 0 to start the timer without blocking.

4fill in blank

hard

Fill both blanks to create a one-shot timer and start it without blocking.

FreeRTOS

TimerHandle_t timer = xTimerCreate("Timer", [1], [2], NULL, timerCallback);
xTimerStart(timer, 0);

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

ApdMS_TO_TICKS(2000)

BpdTRUE

CpdFALSE

DpdMS_TO_TICKS(1000)

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up period and auto-reload parameters.

The timer period is set to 1000 ms and pdFALSE makes it one-shot.

5fill in blank

hard

Fill all three blanks to create an auto-reload timer, start it, and stop it properly.

FreeRTOS

TimerHandle_t timer = xTimerCreate("Timer", [1], [2], NULL, timerCallback);
BaseType_t started = xTimerStart(timer, [3]);
if(started == pdPASS) {
    // Timer started successfully
}

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

ApdMS_TO_TICKS(500)

BpdTRUE

C0

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE for auto-reload when an auto-reload timer is needed.

The timer period is 500 ms, auto-reload is enabled with pdTRUE, and start is called with 0 block time.

Practice

1. What is the main difference between a one-shot timer and an auto-reload timer in FreeRTOS?

easy

A. One-shot timers run once after a delay; auto-reload timers run repeatedly at fixed intervals.

B. One-shot timers run repeatedly; auto-reload timers run only once.

C. One-shot timers are faster than auto-reload timers.

D. Auto-reload timers cannot be stopped once started.

Solution

1. Step 1: Understand timer behavior

   One-shot timers execute their callback function only once after the specified delay.

2. Step 2: Compare with auto-reload timers

   Auto-reload timers automatically restart after each expiration, running repeatedly at fixed intervals.

3. Final Answer:

   One-shot timers run once after a delay; auto-reload timers run repeatedly at fixed intervals. -> Option A

4. Quick Check:

   One-shot = single run, Auto-reload = repeated runs [OK]

Hint: One-shot = once, Auto-reload = repeat [OK]

Common Mistakes:

• Confusing which timer runs repeatedly
• Thinking one-shot timers repeat automatically
• Assuming auto-reload timers cannot be stopped

2. Which of the following is the correct way to create an auto-reload timer in FreeRTOS using xTimerCreate?

easy

A. xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdFALSE, NULL, TimerCallback);

B. xTimerCreate("Timer", 1000, pdTRUE, NULL, TimerCallback);

C. xTimerCreate("Timer", 1000, pdFALSE, NULL, TimerCallback);

D. xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback);

Solution

1. Step 1: Identify auto-reload parameter

   The third parameter of xTimerCreate is uxAutoReload, which should be pdTRUE for auto-reload timers.

2. Step 2: Check time parameter format

   Use pdMS_TO_TICKS(1000) to convert milliseconds to ticks correctly.

3. Final Answer:

   xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback); -> Option D

4. Quick Check:

   Auto-reload needs pdTRUE and tick conversion [OK]

Hint: Auto-reload = pdTRUE, use pdMS_TO_TICKS for time [OK]

Common Mistakes:

• Using pdFALSE for auto-reload timers
• Passing raw milliseconds instead of ticks
• Confusing parameter order

3. Consider this FreeRTOS timer code snippet:

TimerHandle_t timer = xTimerCreate("MyTimer", pdMS_TO_TICKS(500), pdFALSE, NULL, TimerCallback);
xTimerStart(timer, 0);

What will happen when this timer expires?

medium

A. The TimerCallback function will be called once after 500 ms.

B. The TimerCallback function will be called repeatedly every 500 ms.

C. The timer will never start because pdFALSE is invalid.

D. The TimerCallback function will be called immediately and repeatedly.

Solution

1. Step 1: Analyze timer creation parameters

   The timer is created with pdFALSE for auto-reload, meaning it is a one-shot timer.

2. Step 2: Understand timer start and expiration

   After starting, the timer waits 500 ms, then calls TimerCallback once and stops.

3. Final Answer:

   The TimerCallback function will be called once after 500 ms. -> Option A

4. Quick Check:

   pdFALSE means one-shot = single callback [OK]

Hint: pdFALSE means one-shot, callback runs once [OK]

Common Mistakes:

• Thinking pdFALSE means repeated calls
• Assuming timer starts immediately
• Confusing pdFALSE with invalid parameter

4. You wrote this code to create an auto-reload timer but it only runs once:

TimerHandle_t timer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdFALSE, NULL, TimerCallback);
xTimerStart(timer, 0);

What is the likely error?

medium

A. The timer period is too short to see repeated calls.

B. xTimerStart was called with zero block time.

C. The auto-reload parameter is set to pdFALSE instead of pdTRUE.

D. The timer callback function is missing.

Solution

1. Step 1: Check auto-reload parameter

   The timer was created with pdFALSE, which means it is a one-shot timer.

2. Step 2: Understand effect on timer behavior

   Because of pdFALSE, the timer runs only once and does not auto-reload.

3. Final Answer:

   The auto-reload parameter is set to pdFALSE instead of pdTRUE. -> Option C

4. Quick Check:

   pdFALSE disables auto-reload, causing single run [OK]

Hint: Auto-reload needs pdTRUE, not pdFALSE [OK]

Common Mistakes:

• Ignoring auto-reload parameter
• Thinking block time affects reload
• Assuming callback absence causes single run

5. You want to toggle an LED every 2 seconds indefinitely using a FreeRTOS timer. Which timer type and setup should you use?

hard

A. Create a one-shot timer with 2000 ms period and restart it manually each time.

B. Create an auto-reload timer with 2000 ms period to toggle LED automatically.

C. Create a one-shot timer with 1000 ms period and toggle LED twice per cycle.

D. Create an auto-reload timer with 1000 ms period and toggle LED every 2 cycles.

Solution

1. Step 1: Identify requirement for repeated action

   Toggling LED every 2 seconds indefinitely means the action repeats automatically.

2. Step 2: Choose timer type and period

   An auto-reload timer with 2000 ms period will call the toggle function every 2 seconds without manual restart.

3. Final Answer:

   Create an auto-reload timer with 2000 ms period to toggle LED automatically. -> Option B

4. Quick Check:

   Repeated action = auto-reload timer with correct period [OK]

Hint: Repeated tasks use auto-reload timers with correct period [OK]

Common Mistakes:

• Using one-shot timer and forgetting to restart
• Setting wrong timer period
• Confusing toggle frequency with timer period