Task states (Ready, Running, Blocked, Suspended) in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Ready state mean for a FreeRTOS task?

The Ready state means the task is prepared to run but is waiting for the CPU to become available. It is not blocked or suspended.

beginner

Describe the Running state in FreeRTOS.

The Running state means the task is currently executing on the CPU.

beginner

What causes a task to enter the Blocked state?

A task enters the Blocked state when it is waiting for an event, such as a delay to finish or a resource to become available.

intermediate

Explain the Suspended state in FreeRTOS.

A Suspended task is one that has been stopped by the programmer and will not run until explicitly resumed.

intermediate

How does FreeRTOS decide which task moves from Ready to Running?

FreeRTOS uses a scheduler that selects the highest priority task in the Ready state to run next on the CPU.

Which FreeRTOS task state means the task is waiting for a resource or event?

A Blocked

B Ready

C Running

D Suspended

Blocked means the task is waiting for something like a delay or resource.

What state is a task in when it is actively using the CPU?

A Ready

B Blocked

C Running

D Suspended

Running means the task is currently executing on the CPU.

Which state means a task will not run until resumed by the programmer?

A Ready

B Suspended

C Blocked

D Running

Suspended tasks are stopped and need explicit resume to run again.

What determines which Ready task becomes Running in FreeRTOS?

A Task priority

B Task creation order

C Random selection

D Task name alphabetical order

The scheduler picks the highest priority Ready task to run.

If a task is waiting for a timer to expire, what state is it in?

A Running

B Suspended

C Ready

D Blocked

Waiting for a timer means the task is Blocked.

Explain the four main FreeRTOS task states and give a simple example of when a task might be in each state.

Think about what the task is doing or waiting for in each state.

How does FreeRTOS decide which task to run next from the Ready state?

Focus on the role of the scheduler and priorities.

Practice

1.

Which FreeRTOS task state means the task is waiting to be assigned CPU time but is ready to run?

easy

A. Suspended

B. Running

C. Blocked

D. Ready

Solution

1. Step 1: Understand the meaning of Ready state

   The Ready state means the task is prepared and waiting for CPU time but is not currently running.

2. Step 2: Compare with other states

   Running means the task is currently executing. Blocked means waiting for an event. Suspended means paused by user.

3. Final Answer:

   Ready -> Option D

4. Quick Check:

   Ready = waiting to run [OK]

Hint: Ready means waiting but able to run soon [OK]

Common Mistakes:

• Confusing Ready with Running
• Thinking Blocked means ready
• Mixing Suspended with Blocked

2.

Which of the following is the correct way to suspend a task in FreeRTOS?

void suspendTask(TaskHandle_t task) {
    // What function to call?
}

easy

A. vTaskBlock(task);

B. vTaskSuspend(task);

C. vTaskReady(task);

D. vTaskRun(task);

Solution

1. Step 1: Recall FreeRTOS API for suspending tasks

   The function to suspend a task is vTaskSuspend(), which pauses the task until resumed.

2. Step 2: Verify other options

   vTaskBlock() and vTaskReady() do not exist as API calls. vTaskRun() is not a FreeRTOS function.

3. Final Answer:

   vTaskSuspend(task); -> Option B

4. Quick Check:

   Suspend task = vTaskSuspend() [OK]

Hint: Suspend tasks with vTaskSuspend() function [OK]

Common Mistakes:

• Using non-existent functions
• Confusing suspend with block
• Trying to run tasks manually

3.

Consider this FreeRTOS code snippet:

void vTaskFunction(void *pvParameters) {
    for (;;) {
        if (xSemaphoreTake(xSemaphore, 0) == pdTRUE) {
            // Critical section
        } else {
            vTaskDelay(pdMS_TO_TICKS(100));
        }
    }
}

What is the state of the task when vTaskDelay() is called?

medium

A. Running

B. Ready

C. Blocked

D. Suspended

Solution

1. Step 1: Understand vTaskDelay effect

   Calling vTaskDelay causes the task to enter Blocked state, waiting for the delay time to expire.

2. Step 2: Confirm task state during delay

   While delayed, the task is not Ready or Running; it is Blocked until the timer expires.

3. Final Answer:

   Blocked -> Option C

4. Quick Check:

   vTaskDelay = Blocked state [OK]

Hint: vTaskDelay puts task in Blocked state [OK]

Common Mistakes:

• Thinking delay means Ready
• Confusing Blocked with Suspended
• Assuming task runs during delay

4.

Identify the error in this FreeRTOS task management code:

vTaskSuspend(NULL);
vTaskResume(NULL);
vTaskResume(xTaskHandle);

What is wrong with this code?

medium

A. vTaskResume(NULL) is invalid and causes error

B. Cannot suspend current task with NULL parameter

C. vTaskResume requires task handle, NULL is allowed

D. No errors, code is correct

Solution

1. Step 1: Check vTaskSuspend(NULL) usage

   Passing NULL to vTaskSuspend suspends the current task, which is valid.

2. Step 2: Check vTaskResume(NULL) usage

   Passing NULL to vTaskResume is invalid and causes an error because NULL is not a valid task handle to resume.

3. Final Answer:

   vTaskResume(NULL) is invalid and causes error -> Option A

4. Quick Check:

   vTaskResume(NULL) causes error [OK]

Hint: vTaskResume(NULL) is invalid, must use valid handle [OK]

Common Mistakes:

• Assuming NULL resumes current task
• Confusing suspend and resume parameters
• Ignoring NULL parameter rules

5.

You want to create a task that waits for a button press event without using CPU time unnecessarily. Which task state should the task enter while waiting?

hard

A. Blocked

B. Ready

C. Running

D. Suspended

Solution

1. Step 1: Understand waiting without CPU use

   To avoid wasting CPU, the task should not be Running or Ready while waiting.

2. Step 2: Choose appropriate waiting state

   Blocked state is used when a task waits for an event like a button press, so it sleeps until event occurs.

3. Step 3: Differentiate from Suspended

   Suspended means paused by user, not event-driven waiting.

4. Final Answer:

   Blocked -> Option A

5. Quick Check:

   Wait for event = Blocked state [OK]

Hint: Wait for event = Blocked to save CPU [OK]

Common Mistakes:

• Using Suspended instead of Blocked
• Keeping task Ready and wasting CPU
• Confusing Running with waiting