Common embedded bugs and fixes in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to correctly initialize the pointer.

Embedded C

int value = 10;
int *ptr = [1];

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

A&value

B*value

Cvalue

Dptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using the variable name directly instead of its address.

Dereferencing the variable instead of taking its address.

The pointer must be initialized with the address of the variable using the & operator.

2fill in blank

medium

Complete the code to correctly check if a bit is set in a register.

Embedded C

if ((reg & [1]) != 0) {
    // bit is set
}

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A0x01

B1

C0x00

Dreg

Attempts:

3 left

💡 Hint

Common Mistakes

Using the register itself as a mask.

Using zero as a mask which always results false.

Use a mask with the bit set (0x01) to check the first bit in the register.

3fill in blank

hard

Fix the error in the interrupt service routine declaration.

Embedded C

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

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Astatic

Binterrupt

Cvoid

D__interrupt

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'interrupt' instead of '__interrupt'.

Omitting the interrupt keyword entirely.

In embedded C, the ISR function must be declared with the __interrupt keyword to be recognized as an interrupt handler.

4fill in blank

hard

Fill both blanks to correctly define a volatile pointer to a hardware register.

Embedded C

volatile [1] *[2] = (volatile [1] *)0x40021000;

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Auint32_t

Breg

Cint

Dptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' instead of 'uint32_t' which may have wrong size.

Using non-volatile types which can cause optimization issues.

The hardware register is usually accessed as a volatile uint32_t pointer. The pointer variable can be named 'ptr'.

5fill in blank

hard

Fill all three blanks to correctly implement a delay loop using a volatile counter.

Embedded C

void delay(void) {
    volatile int [1] = [2];
    while ([1] > 0) {
        [1]--;
    }
}

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

Acounter

B10000

Ci

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-volatile variable which the compiler may optimize away.

Initializing the counter to zero which causes no delay.

The volatile variable 'counter' is initialized to 10000 and decremented until zero to create a delay.

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