ISR best practices (keep it short) in Embedded C - Step-by-Step Execution

Concept Flow - ISR best practices (keep it short)

Interrupt Occurs

↓

Save Context

↓

Execute ISR Quickly

↓

Clear Interrupt Flag

↓

Restore Context

↓

Return to Main Program

When an interrupt happens, save the current state, run the ISR fast, clear the interrupt, restore state, then continue main code.

Execution Sample

Embedded C

void ISR_Handler() {
  save_context();
  // quick ISR code
  clear_interrupt_flag();
  restore_context();
}

This ISR saves context, runs minimal code, clears interrupt, and restores context.

Execution Table

Step	Action	Details	Reason
1	Interrupt Occurs	CPU stops main code	Respond to event immediately
2	Save Context	Store registers and state	Preserve main program state
3	Execute ISR Quickly	Run minimal code	Avoid delaying main program
4	Clear Interrupt Flag	Reset interrupt signal	Prevent repeated triggers
5	Restore Context	Reload saved registers	Return to main program smoothly
6	Return to Main Program	Resume normal execution	Continue main tasks

💡 ISR ends after restoring context and clearing interrupt flag

Variable Tracker

Variable	Start	After Save Context	After ISR Code	After Restore Context	Final
CPU Registers	Main program values	Saved to stack	Modified if needed	Restored from stack	Main program values
Interrupt Flag	Set (1)	Set (1)	Cleared (0)	Cleared (0)	Cleared (0)

Key Moments - 3 Insights

Why must ISR code be short and fast?

Because long ISRs block main code and other interrupts, slowing system response (see execution_table step 3).

What happens if the interrupt flag is not cleared?

The ISR may run repeatedly causing a loop, as shown in execution_table step 4.

Why save and restore CPU context in ISR?

To keep main program running correctly after ISR finishes, as shown in steps 2 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the interrupt flag cleared?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Check the 'Clear Interrupt Flag' action in execution_table step 4

According to variable_tracker, what happens to CPU registers after ISR code runs?

A They are restored to main program values

B They are saved to stack

C They remain modified

D They are cleared

💡 Hint

Look at 'CPU Registers' row in the 'After ISR Code' column

If ISR code is long, what is the likely effect on the main program?

A Main program runs faster

B Main program is blocked longer

C Interrupt flag clears automatically

D CPU context is not saved

💡 Hint

Refer to key_moments about ISR speed and execution_table step 3

Concept Snapshot

ISR Best Practices:
- Save CPU context at start
- Keep ISR code short and fast
- Clear interrupt flag inside ISR
- Restore CPU context before return
- Avoid blocking main program

Full Transcript

When an interrupt occurs, the CPU stops the main program and saves its current state. The ISR runs quickly to handle the event, then clears the interrupt flag to avoid repeated triggers. After that, the CPU restores the saved state and resumes the main program. Keeping ISR code short prevents delays and ensures smooth system operation.