ISR best practices (keep it short) in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an ISR function correctly.

Embedded C

void [1](void) {
    // ISR code here
}

Drag options to blanks, or click blank then click option'

Ainterrupt

BISR

CISR_Handler

Dmain

Attempts:

3 left

💡 Hint

Common Mistakes

Using normal function names like 'main' or 'interrupt' without proper ISR declaration.

The ISR function should be declared with the correct name or attribute depending on the platform. Here, ISR is the correct placeholder for the interrupt service routine.

2fill in blank

medium

Complete the code to keep the ISR short by only setting a flag.

Embedded C

volatile int flag = 0;

void ISR(void) {
    [1] = 1;
}

Drag options to blanks, or click blank then click option'

Aflag

Bresult

Ctemp

Dcounter

Attempts:

3 left

💡 Hint

Common Mistakes

Modifying non-volatile variables inside ISR.

In ISR best practices, keep the ISR short by setting a flag variable to signal the main program.

3fill in blank

hard

Fix the error in the ISR by disabling nested interrupts.

Embedded C

void ISR(void) {
    [1]();
    // ISR code
}

Drag options to blanks, or click blank then click option'

Aenable_nested_interrupts

Benable_interrupts

Cdisable_interrupts

Dclear_interrupt_flag

Attempts:

3 left

💡 Hint

Common Mistakes

Enabling interrupts inside ISR causing nested calls.

To avoid nested interrupts causing issues, disable interrupts at the start of the ISR.

4fill in blank

hard

Fill both blanks to safely update a shared variable in ISR and main code.

Embedded C

volatile int count = 0;

void ISR(void) {
    [1]();
    count++;
    [2]();
}

Drag options to blanks, or click blank then click option'

Adisable_interrupts

Benable_interrupts

Cclear_interrupt_flag

Dreset_counter

Attempts:

3 left

💡 Hint

Common Mistakes

Not disabling interrupts leading to data corruption.

Disable interrupts before updating shared variables and enable them after to avoid race conditions.

5fill in blank

hard

Fill all three blanks to implement a flag check and clear in main loop and ISR.

Embedded C

volatile int flag = 0;

void ISR(void) {
    [1] = 1;
}

int main(void) {
    while(1) {
        if ([2] == 1) {
            // handle event
            [3] = 0;
        }
    }
}

Drag options to blanks, or click blank then click option'

Aflag

Bevent

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names causing logic errors.

The ISR sets the flag to 1. The main loop checks the flag and clears it after handling the event.

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