Common embedded bugs and fixes in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a common cause of a 'watchdog timer reset' in embedded systems?

A watchdog timer reset often happens when the system gets stuck in an infinite loop or deadlock and fails to reset the watchdog timer in time.

beginner

Why can uninitialized variables cause bugs in embedded C programs?

Uninitialized variables contain random data, which can cause unpredictable behavior or crashes when used before being set.

intermediate

How can stack overflow occur in embedded systems?

Stack overflow happens when too much memory is used on the stack, often due to deep or infinite recursion or large local variables.

intermediate

What is a common fix for race conditions in embedded systems?

Use synchronization methods like disabling interrupts or using mutexes to protect shared resources and prevent race conditions.

beginner

Why is it important to check return values of hardware access functions?

Checking return values helps detect hardware errors early and prevents the program from continuing with invalid data or states.

What is a typical symptom of a stack overflow in an embedded system?

A System crashes or behaves unpredictably

B Faster program execution

C Increased battery life

D Improved memory usage

Stack overflow causes memory corruption leading to crashes or unpredictable behavior.

Which of the following can help prevent race conditions?

A Disabling interrupts during critical sections

B Ignoring shared variables

C Using global variables without protection

D Increasing clock speed

Disabling interrupts during critical code sections prevents concurrent access and race conditions.

Why should you initialize variables before use in embedded C?

A To save memory

B To avoid unpredictable behavior from garbage values

C To make the program run slower

D To increase code size

Uninitialized variables hold random data that can cause bugs or crashes.

What does a watchdog timer do in embedded systems?

A Increases CPU speed

B Monitors battery level

C Saves power

D Resets the system if it becomes unresponsive

A watchdog timer resets the system if it stops responding to prevent hangs.

What is a good practice when accessing hardware registers?

A Use random values

B Ignore errors and continue

C Check return values for errors

D Access registers without delay

Checking return values helps detect hardware faults early.

Describe three common bugs in embedded C programming and how to fix them.

Think about memory issues, concurrency, and variable initialization.

Explain why watchdog timers are used and what causes them to reset the system.

Consider system hangs and how watchdogs help recover.

Practice

1. What is a common cause of unpredictable behavior in embedded C programs related to variables?

easy

A. Using uninitialized variables before setting a value

B. Using too many comments in the code

C. Writing functions with too many parameters

D. Using global variables only

Solution

1. Step 1: Understand variable initialization

   Uninitialized variables contain garbage values, causing unpredictable behavior.

2. Step 2: Recognize the impact in embedded systems

   Embedded systems rely on precise values; uninitialized variables can cause errors or crashes.

3. Final Answer:

   Using uninitialized variables before setting a value -> Option A

4. Quick Check:

   Uninitialized variables = unpredictable values [OK]

Hint: Always initialize variables before use to avoid surprises [OK]

Common Mistakes:

• Assuming variables start at zero automatically
• Ignoring compiler warnings about uninitialized variables
• Thinking only pointers need initialization

2. Which of the following is the correct way to clear an interrupt flag in embedded C?

easy

A. interrupt_flag++;

B. interrupt_flag = 1;

C. interrupt_flag = 0;

D. interrupt_flag = -1;

Solution

1. Step 1: Identify interrupt flag clearing method

   Interrupt flags are cleared by setting them to 0 to indicate the interrupt is handled.

2. Step 2: Understand why other options fail

   Setting to 1 or incrementing does not clear the flag; negative values are invalid.

3. Final Answer:

   interrupt_flag = 0; -> Option C

4. Quick Check:

   Clear interrupt flag = 0 [OK]

Hint: Clear interrupt flags by setting them to zero [OK]

Common Mistakes:

• Setting flag to 1 instead of 0
• Incrementing flag instead of clearing
• Ignoring the need to clear flags causing repeated interrupts

3. What will be the output of this embedded C code snippet?

volatile int flag = 0;
void ISR() {
  flag = 1;
}
int main() {
  while(flag == 0) {
    // wait
  }
  return 0;
}

medium

A. Program waits until ISR sets flag to 1, then exits

B. Program exits immediately

C. Compilation error due to volatile keyword

D. Program runs indefinitely without stopping

Solution

1. Step 1: Understand volatile keyword usage

   Volatile tells compiler the variable can change anytime, so it won't optimize the loop away.

2. Step 2: Analyze program flow

   Main waits in loop until ISR sets flag to 1, then loop ends and program exits.

3. Final Answer:

   Program waits until ISR sets flag to 1, then exits -> Option A

4. Quick Check:

   Volatile flag updated by ISR = loop ends [OK]

Hint: Use volatile for variables changed by interrupts [OK]

Common Mistakes:

• Ignoring volatile causing infinite loop
• Assuming ISR runs automatically without setup
• Thinking volatile causes compile errors

4. Identify the bug in this embedded C code and the best fix:

int counter;
void setup() {
  // No initialization of counter
}
void loop() {
  counter++;
  if(counter > 10) {
    counter = 0;
  }
}

medium

A. Change counter to volatile

B. Initialize counter to 0 in setup()

C. Remove the if condition

D. Declare counter as static

Solution

1. Step 1: Identify uninitialized variable usage

   Counter is used without initialization, causing unpredictable start value.

2. Step 2: Fix by initializing counter

   Set counter = 0 in setup() to ensure predictable counting behavior.

3. Final Answer:

   Initialize counter to 0 in setup() -> Option B

4. Quick Check:

   Initialize variables before use = correct count [OK]

Hint: Always initialize counters before incrementing [OK]

Common Mistakes:

• Assuming global variables start at zero
• Using volatile without need
• Ignoring initialization causing wrong counts

5. You have a sensor reading stored in a variable that updates via interrupt. Which combination of fixes ensures safe and correct reading in main code?

volatile int sensor_value;
int safe_value;

void ISR() {
  sensor_value = read_sensor();
}

int main() {
  // What should be done here?
}

hard

A. Make sensor_value non-volatile and copy it

B. Directly use sensor_value without copying

C. Use a delay loop before reading sensor_value

D. Disable interrupts, copy sensor_value to safe_value, enable interrupts

Solution

1. Step 1: Understand volatile and interrupt safety

   Volatile ensures compiler reads fresh value, but reading multi-byte data can be interrupted causing corruption.

2. Step 2: Use interrupt disable-copy-enable pattern

   Disable interrupts, copy sensor_value to safe_value, then enable interrupts to get consistent snapshot.

3. Final Answer:

   Disable interrupts, copy sensor_value to safe_value, enable interrupts -> Option D

4. Quick Check:

   Disable interrupts to safely copy volatile data [OK]

Hint: Disable interrupts when copying multi-byte volatile data [OK]

Common Mistakes:

• Reading volatile multi-byte data without protection
• Making volatile variable non-volatile
• Using delay instead of disabling interrupts