Hard real-time vs soft real-time in FreeRTOS - Visual Side-by-Side Comparison

Concept Flow - Hard real-time vs soft real-time

Task starts

↓

Deadline check

↓

Deadline met?

↓

Continue normal

↓

Next task

↓

Hard RT: system fail

Tasks start and check deadlines. If met, continue normally. If missed, soft real-time systems degrade gracefully, hard real-time systems fail immediately.

Execution Sample

FreeRTOS

void Task(void *params) {
  TickType_t start = xTaskGetTickCount();
  // Do work
  if (xTaskGetTickCount() - start > DEADLINE) {
    // Deadline missed
  }
}

This code checks if a task finishes before its deadline and handles deadline misses.

Execution Table

Step	Action	Time (ticks)	Deadline (ticks)	Deadline Met?	System Response
1	Task starts	100	50	N/A	Running
2	Task does work	160	50	N/A	Running
3	Check deadline	160	50	No (160-100=60 > 50?)	Deadline missed
4	Soft RT response	160	50	No	Continue with degraded quality
5	Hard RT response	160	50	No	System fails or resets

💡 Execution stops after deadline check and system response to deadline miss.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
start	100	100	100	100
current_time	N/A	160	160	160
deadline	50	50	50	50
deadline_met	N/A	N/A	No	No

Key Moments - 2 Insights

Why does the system fail immediately in hard real-time but not in soft real-time?

Because in hard real-time (see execution_table step 5), missing a deadline is critical and causes failure, while soft real-time (step 4) allows continuing with degraded performance.

How is the deadline checked in the code?

By comparing the elapsed time (current_time - start) with the deadline value, as shown in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the deadline checked?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Action' column for 'Check deadline' in execution_table.

According to variable_tracker, what is the value of 'deadline_met' after step 3?

A No

B Yes

C N/A

D Undefined

💡 Hint

Look at the 'deadline_met' row and the 'After Step 3' column in variable_tracker.

If the task finishes before the deadline, what would the system response be in a hard real-time system?

A Degrade quality

B System fails

C Continue normal operation

D Reset system

💡 Hint

Refer to concept_flow where meeting the deadline leads to normal continuation.

Concept Snapshot

Hard real-time: Missing deadlines causes system failure.
Soft real-time: Missing deadlines causes degraded performance.
Check deadlines by comparing elapsed time to deadline.
Hard RT is strict; soft RT is flexible.
Use FreeRTOS tick counts to measure time.

Full Transcript

In real-time systems, tasks must finish work before deadlines. Hard real-time systems fail immediately if deadlines are missed. Soft real-time systems allow continuing with reduced quality. The code example shows checking elapsed time against a deadline. Execution steps trace task start, work, deadline check, and system response. Variables track start time, current time, deadline, and if deadline was met. Key moments clarify why hard RT fails on misses and how deadlines are checked. Quizzes test understanding of steps and variable values. Remember: hard RT is strict, soft RT is forgiving.

Practice

1. Which statement best describes a hard real-time system in FreeRTOS?

easy

A. Tasks can miss deadlines occasionally without major issues.

B. Tasks run only when the CPU is idle.

C. Tasks must meet deadlines strictly without any misses.

D. Tasks have no timing constraints.

Solution

1. Step 1: Understand hard real-time definition

   Hard real-time means deadlines are strict and must never be missed.

2. Step 2: Compare options with definition

   The statement 'Tasks must meet deadlines strictly without any misses' matches the definition of hard real-time.

3. Final Answer:

   Tasks must meet deadlines strictly without any misses. -> Option C

4. Quick Check:

   Hard real-time = strict deadlines [OK]

Hint: Hard real-time means no deadline misses allowed [OK]

Common Mistakes:

• Confusing hard real-time with soft real-time
• Thinking tasks run only when CPU is idle
• Ignoring timing constraints

2. Which FreeRTOS API function is used to delay a task for a specific time period?

easy

A. vTaskDelay()

B. xTaskCreate()

C. vTaskStartScheduler()

D. xQueueSend()

Solution

1. Step 1: Identify delay function in FreeRTOS

   vTaskDelay() pauses a task for a given tick count, controlling timing.

2. Step 2: Check other options

   xTaskCreate() creates tasks, vTaskStartScheduler() starts the OS, xQueueSend() sends to queues.

3. Final Answer:

   vTaskDelay() -> Option A

4. Quick Check:

   Delay task = vTaskDelay() [OK]

Hint: Delay tasks with vTaskDelay() in FreeRTOS [OK]

Common Mistakes:

• Confusing task creation with delay
• Using scheduler start as delay
• Mixing queue functions with delays

3. Consider this FreeRTOS task priority setup:

Task A priority = 3
Task B priority = 2
Task C priority = 1

If Task A blocks for 100ms using vTaskDelay(), which task runs next?

medium

A. Task A again immediately

B. Task B

C. Task C

D. No task runs until Task A finishes delay

Solution

1. Step 1: Understand task priorities and blocking

   Task A has highest priority (3) but blocks for 100ms, so it is not ready.

2. Step 2: Identify next highest priority ready task

   Task B has priority 2 and is ready, so it runs next.

3. Final Answer:

   Task B -> Option B

4. Quick Check:

   Next task after blocking = highest ready priority [OK]

Hint: Next runs highest priority ready task after delay [OK]

Common Mistakes:

• Assuming blocked task runs immediately
• Ignoring task priorities
• Thinking no task runs during delay

4. Identify the error in this FreeRTOS code snippet for a hard real-time task:

void vTaskFunction(void *pvParameters) {
  while(1) {
    // Critical processing
    vTaskDelay(1000);
  }
}

What is the problem?

medium

A. vTaskDelay should be replaced with vTaskDelete.

B. The task function lacks a return statement.

C. The infinite loop will crash FreeRTOS.

D. Using vTaskDelay causes deadline misses in hard real-time tasks.

Solution

1. Step 1: Analyze vTaskDelay in hard real-time context

   vTaskDelay pauses the task, which can cause missing strict deadlines in hard real-time.

2. Step 2: Check other options for correctness

   Return statement is not needed in infinite loop, infinite loops are normal, vTaskDelete deletes task, not delay.

3. Final Answer:

   Using vTaskDelay causes deadline misses in hard real-time tasks. -> Option D

4. Quick Check:

   Hard real-time tasks avoid vTaskDelay [OK]

Hint: Hard real-time tasks avoid delays that cause misses [OK]

Common Mistakes:

• Thinking infinite loops crash FreeRTOS
• Confusing vTaskDelay with task deletion
• Expecting return in infinite loops

5. You need to design a FreeRTOS system where some tasks must never miss deadlines (hard real-time), and others can miss occasionally (soft real-time). Which approach best achieves this?

hard

A. Assign highest priorities to hard real-time tasks and use vTaskDelay carefully for soft real-time tasks.

B. Use vTaskDelay for all tasks to simplify timing.

C. Assign equal priorities to all tasks and rely on round-robin scheduling.

D. Run hard real-time tasks only when soft real-time tasks are idle.

Solution

1. Step 1: Understand priority and delay roles in FreeRTOS

   Hard real-time tasks need highest priority to avoid preemption and deadline misses.

2. Step 2: Use vTaskDelay carefully for soft real-time tasks

   Soft real-time tasks can tolerate delays, so vTaskDelay helps manage their timing without affecting hard real-time tasks.

3. Step 3: Evaluate other options

   Equal priorities or running hard real-time tasks only when soft are idle do not guarantee deadlines.

4. Final Answer:

   Assign highest priorities to hard real-time tasks and use vTaskDelay carefully for soft real-time tasks. -> Option A

5. Quick Check:

   Priority + careful delay = mixed real-time system [OK]

Hint: Prioritize hard real-time tasks highest, delay soft tasks [OK]

Common Mistakes:

• Giving equal priority to all tasks
• Using delays in hard real-time tasks
• Running hard real-time tasks only after soft tasks