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.

Practice

1. What is the main reason to keep Interrupt Service Routine (ISR) code short in embedded C?

easy

A. To use more memory for ISR variables

B. To enable ISR to call blocking functions safely

C. To allow ISR to run complex calculations

D. To avoid delaying other interrupts and keep system responsive

Solution

1. Step 1: Understand ISR impact on system

   ISRs temporarily stop main code and other interrupts, so long ISRs delay system response.

2. Step 2: Reason why short ISR is important

   Short ISR avoids blocking other interrupts and keeps system fast and responsive.

3. Final Answer:

   To avoid delaying other interrupts and keep system responsive -> Option D

4. Quick Check:

   Keep ISR short = avoid delays [OK]

Hint: Short ISR avoids blocking other interrupts [OK]

Common Mistakes:

• Thinking ISR can run long calculations
• Using blocking calls inside ISR
• Assuming ISR uses more memory for variables

2. Which of the following is the correct way to signal the main program from an ISR?

easy

A. Call a heavy processing function directly inside ISR

B. Print debug messages inside ISR

C. Set a flag variable that main code checks

D. Use a blocking delay inside ISR

Solution

1. Step 1: Identify safe communication from ISR

   Heavy processing or blocking calls inside ISR are unsafe; printing can cause delays.

2. Step 2: Use flag to communicate

   Setting a flag variable is fast and lets main code handle processing safely.

3. Final Answer:

   Set a flag variable that main code checks -> Option C

4. Quick Check:

   Use flags to signal main code [OK]

Hint: Use flags, not heavy calls, inside ISR [OK]

Common Mistakes:

• Calling complex functions inside ISR
• Using delays or blocking calls in ISR
• Printing inside ISR causing slowdowns

3. Given this ISR code snippet, what will be the value of flag after the interrupt triggers once?

volatile int flag = 0;
void ISR() {
  flag = 1;
}

medium

A. 1

B. 0

C. Undefined

D. ISR causes error

Solution

1. Step 1: Understand ISR effect on flag

   ISR sets flag to 1 when triggered.

2. Step 2: Check initial and final flag value

   Initially flag is 0, after ISR runs once, flag becomes 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   ISR sets flag = 1 [OK]

Hint: ISR sets flag to 1 on trigger [OK]

Common Mistakes:

• Assuming flag stays 0
• Thinking ISR causes error
• Confusing volatile keyword effect

4. Identify the problem in this ISR code:

void ISR() {
  while(!data_ready) {
    // wait for data
  }
  process_data();
}

medium

A. Not setting a flag variable

B. Using a blocking loop inside ISR

C. ISR missing return statement

D. Using volatile variable incorrectly

Solution

1. Step 1: Analyze ISR code behavior

   ISR contains a while loop waiting for data_ready, which blocks ISR execution.

2. Step 2: Understand why blocking is bad in ISR

   Blocking inside ISR delays other interrupts and main code, causing system issues.

3. Final Answer:

   Using a blocking loop inside ISR -> Option B

4. Quick Check:

   Blocking loops inside ISR cause problems [OK]

Hint: Avoid loops that wait inside ISR [OK]

Common Mistakes:

• Ignoring blocking loops in ISR
• Thinking ISR needs return statement
• Confusing volatile usage

5. You want to handle a sensor interrupt by reading data and processing it. Which is the best approach following ISR best practices?

Options:
A) Read and process data fully inside ISR
B) Use delay inside ISR to wait for sensor
C) Set a flag in ISR and process data in main loop
D) Print sensor data inside ISR for debugging

hard

A. Set a flag in ISR and process data in main loop

B. Use delay inside ISR to wait for sensor

C. Read and process data fully inside ISR

D. Print sensor data inside ISR for debugging

Solution

1. Step 1: Evaluate ISR workload options

   Processing data fully or using delay inside ISR blocks system; printing slows ISR.

2. Step 2: Choose safe communication method

   Setting a flag in ISR and processing in main loop keeps ISR short and system responsive.

3. Final Answer:

   Set a flag in ISR and process data in main loop -> Option A

4. Quick Check:

   Flag in ISR, process later = best practice [OK]

Hint: Use flags in ISR, process outside [OK]

Common Mistakes:

• Doing heavy work inside ISR
• Using delays or prints in ISR
• Ignoring main loop processing