ISR best practices (keep it short) in Embedded C

Introduction

ISRs handle urgent tasks quickly when hardware signals happen. Following best practices keeps your program fast and reliable.

When a hardware event like a button press or sensor signal needs immediate attention.

When you must respond quickly to data arriving from communication ports.

When you want to avoid missing important signals by checking them often.

When you want to keep your main program running smoothly without delays.

When you need to update shared data safely between ISR and main code.

Syntax

Embedded C

void ISR_Handler(void) {
    // ISR code here
}

Keep ISR code short and fast to avoid blocking other tasks.

Avoid heavy processing or long loops inside ISR.

Examples

Set a simple flag to handle the event later in the main program.

Embedded C

void ISR_ButtonPress(void) {
    buttonPressedFlag = 1; // Set a flag
}

Quickly read data and set a flag for processing outside ISR.

Embedded C

void ISR_UART(void) {
    receivedData = UART_Read();
    dataReadyFlag = 1;
}

Sample Program

This program simulates an ISR setting a flag and the main program checking it to handle the event.

Embedded C

#include <stdio.h>
#include <stdbool.h>

volatile bool buttonPressedFlag = false;

void ISR_ButtonPress(void) {
    buttonPressedFlag = true; // Quick flag set
}

int main() {
    printf("Program started\n");
    // Simulate button press interrupt
    ISR_ButtonPress();

    if (buttonPressedFlag) {
        printf("Button was pressed! Handling event in main.\n");
        buttonPressedFlag = false; // Reset flag
    }

    return 0;
}

Important Notes

Never call slow functions like printf inside ISR.

Use volatile keyword for variables shared between ISR and main code.

Disable interrupts only if absolutely necessary and keep that time very short.

Summary

Keep ISR code short and simple.

Use flags or buffers to communicate with main code.

Avoid heavy processing or blocking calls inside ISR.

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