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.