Nested interrupt handling in FreeRTOS - Step-by-Step Execution

Concept Flow - Nested interrupt handling

Interrupt 1 occurs

↓

Interrupt 1 handler starts

↓

Interrupt 2 occurs (higher priority)

↓

Interrupt 1 paused

↓

Interrupt 2 handler runs

↓

Interrupt 2 handler ends

↓

Interrupt 1 resumes

↓

Interrupt 1 handler ends

↓

Return to main program

When a higher priority interrupt occurs during a lower priority interrupt, the lower one pauses, the higher runs, then the lower resumes.

Execution Sample

FreeRTOS

void ISR1() {
  // Start ISR1
  if (higher_priority_interrupt) {
    ISR2(); // Nested interrupt
  }
  // Finish ISR1
}

void ISR2() {
  // Handle higher priority interrupt
}

Shows ISR1 starting, then ISR2 interrupting it, then ISR1 resuming after ISR2 finishes.

Execution Table

Step	Event	Current ISR	Action	State
1	Interrupt 1 occurs	None	Start ISR1	ISR1 running
2	Interrupt 2 occurs (higher priority)	ISR1	Pause ISR1, start ISR2	ISR1 paused, ISR2 running
3	ISR2 completes	ISR2	End ISR2, resume ISR1	ISR1 running
4	ISR1 completes	ISR1	End ISR1	No ISR running, return to main
5	Main program resumes	None	Continue normal execution	Main running

💡 All interrupts handled, system returns to main program

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
ISR State	None	ISR1 running	ISR1 paused, ISR2 running	ISR1 running	No ISR running	No ISR running

Key Moments - 3 Insights

Why does ISR1 pause when ISR2 starts?

Because ISR2 has higher priority, the system pauses ISR1 to handle ISR2 first, as shown in step 2 of the execution_table.

How does the system know to resume ISR1 after ISR2 finishes?

After ISR2 ends (step 3), the system resumes the paused ISR1 automatically, continuing from where it left off.

What happens if a lower priority interrupt occurs during ISR1?

Lower priority interrupts wait until ISR1 finishes; they do not interrupt ISR1, unlike higher priority interrupts.

Visual Quiz - 3 Questions

Look at the execution_table, what is the ISR state after step 2?

A ISR1 running, ISR2 paused

B ISR1 paused, ISR2 running

C No ISR running

D ISR2 paused, ISR1 running

💡 Hint

Check the 'State' column in row for step 2 in execution_table

At which step does ISR1 resume after being paused?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for when ISR1 resumes

If ISR2 had lower priority than ISR1, what would happen at step 2?

A ISR2 waits until ISR1 finishes

B Both ISR1 and ISR2 run simultaneously

C ISR1 pauses and ISR2 runs

D ISR2 cancels ISR1

💡 Hint

Recall that only higher priority interrupts can preempt lower ones, see key_moments explanation

Concept Snapshot

Nested interrupt handling allows higher priority interrupts to pause lower priority ones.
Lower priority interrupts wait until current ISR finishes.
ISR state changes: running -> paused -> resumed.
System returns to main program after all ISRs complete.
This ensures urgent tasks get immediate attention.

Full Transcript

Nested interrupt handling means when a higher priority interrupt happens during a lower priority interrupt, the lower one pauses and the higher one runs. After the higher priority interrupt finishes, the lower priority interrupt resumes. This process ensures urgent tasks are handled immediately without losing progress on the interrupted task. The execution table shows step-by-step how ISR1 starts, ISR2 interrupts it, ISR2 finishes, then ISR1 resumes and finishes, returning control to the main program.