Watching register values in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read the value from the register REG_A.

Embedded C

uint8_t value = [1];

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

A*REG_A

BREG_A

C&REG_A

DREG_A()

Attempts:

3 left

💡 Hint

Common Mistakes

Using REG_A without dereferencing returns the pointer, not the value.

Using ®_A gives the address of the pointer, not the register value.

To read the value stored at the memory address of REG_A, you need to dereference the pointer using *REG_A.

2fill in blank

medium

Complete the code to write the value 0xFF to the register REG_B.

Embedded C

[1] = 0xFF;

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

AREG_B

BREG_B()

C&REG_B

D*REG_B

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning to REG_B directly changes the pointer, not the register value.

Using ®_B is the address of the pointer, not the register.

To write a value to the hardware register, you must dereference the pointer REG_B and assign the value.

3fill in blank

hard

Fix the error in the code to toggle bit 3 of the register REG_C.

Embedded C

*REG_C = *REG_C [1] (1 << 3);

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

A&

B^

C|

D<<

Attempts:

3 left

💡 Hint

Common Mistakes

Using & clears bits instead of toggling.

Using | sets bits but does not toggle.

Using << is a shift operator, not a bitwise toggle.

To toggle a bit, use the XOR operator ^ with a mask that has that bit set.

4fill in blank

hard

Fill both blanks to check if bit 5 of REG_D is set and clear it if true.

Embedded C

if ((*REG_D [1] (1 << 5)) != 0) {
    *REG_D [2]= ~(1 << 5);
}

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

A&

B|=

C&=

D^=

Attempts:

3 left

💡 Hint

Common Mistakes

Using |= sets bits instead of clearing.

Using ^= toggles bits instead of clearing.

Use & to check if bit 5 is set. Use &= ~ to clear that bit.

5fill in blank

hard

Fill all three blanks to create a mask for bits 0 to 3, set those bits in REG_E, and then read the masked value.

Embedded C

uint8_t mask = (1 [1] 4) - 1;
*REG_E [2]= mask;
uint8_t val = *REG_E [3] mask;

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

A<<

B|=

C&

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using ^ to set bits toggles them instead.

Using & to set bits clears other bits.

Shift 1 left by 4 to create mask bits 0-3. Use |= to set bits. Use & to read masked bits.

Practice

1. What is the main purpose of watching register values in embedded C debugging?

easy

A. To compile the program faster

B. To modify register values directly in the code

C. To observe hardware changes during program execution

D. To write comments about hardware

Solution

1. Step 1: Understand what watching registers means

   Watching register values means monitoring hardware registers as the program runs to see changes.

2. Step 2: Identify the purpose of watching registers

   This helps debug hardware interactions by observing real-time changes, not modifying code or compiling.

3. Final Answer:

   To observe hardware changes during program execution -> Option C

4. Quick Check:

   Watching registers = observe hardware changes [OK]

Hint: Watching means observing hardware changes live, not changing code [OK]

Common Mistakes:

• Thinking watching modifies registers in code
• Confusing watching with faster compilation
• Assuming watching adds comments

2. Which of the following is the correct way to watch a register value in a debugger for embedded C?

easy

A. Write code to print the register value every second

B. Add the register's address or name from device headers to the watch list

C. Use printf to display register values in main()

D. Declare the register as a global variable

Solution

1. Step 1: Recall how watches are set in debugging

   Watches are set in the debugger, not by code changes, using register addresses or names.

2. Step 2: Identify the correct method

   Adding the register's address or name from device headers to the watch list is the correct way.

3. Final Answer:

   Add the register's address or name from device headers to the watch list -> Option B

4. Quick Check:

   Watch = add register address/name in debugger [OK]

Hint: Use debugger watch list with register address/name, not code [OK]

Common Mistakes:

• Trying to watch by adding print statements
• Confusing code changes with debugger watches
• Declaring registers as variables incorrectly

3. Given the code snippet below, what will you see if you watch the register PORTA during execution?

PORTA = 0x00;
PORTA |= 0x01;
PORTA &= ~0x02;

medium

A. PORTA changes from 0x00 to 0x02

B. PORTA changes from 0x00 to 0x03

C. PORTA remains 0x00 throughout

D. PORTA changes from 0x00 to 0x01 and stays 0x01

Solution

1. Step 1: Analyze initial assignment

   PORTA is set to 0x00 (all bits cleared).

2. Step 2: Apply bitwise OR operation

   PORTA |= 0x01 sets bit 0, so PORTA becomes 0x01.

3. Step 3: Apply bitwise AND with complement

   PORTA &= ~0x02 clears bit 1, but bit 1 was already 0, so PORTA stays 0x01.

4. Final Answer:

   PORTA changes from 0x00 to 0x01 and stays 0x01 -> Option D

5. Quick Check:

   Bitwise OR sets bit 0, AND clears bit 1 (already 0) [OK]

Hint: Track bitwise operations step-by-step on register value [OK]

Common Mistakes:

• Assuming bit 1 was set initially
• Confusing OR and AND operations
• Ignoring initial register value

4. You added a watch on register TIMER_CTRL but it never updates during debugging. What is the most likely cause?

medium

A. The watch was added incorrectly using a wrong address or name

B. The register is read-only and cannot be watched

C. The debugger does not support watching registers

D. The program has a syntax error

Solution

1. Step 1: Understand watch setup requirements

   Watches must use correct register addresses or names from device headers.

2. Step 2: Identify why watch doesn't update

   If the watch never updates, it is likely because the address or name used is incorrect.

3. Final Answer:

   The watch was added incorrectly using a wrong address or name -> Option A

4. Quick Check:

   Wrong address/name = no register updates in watch [OK]

Hint: Verify register address/name exactly when adding watch [OK]

Common Mistakes:

• Assuming read-only registers can't be watched
• Blaming debugger without checking watch setup
• Confusing syntax errors with watch issues

5. You want to watch only bit 3 of the STATUS_REG register to detect a flag change. Which approach is correct?

hard

A. Add a watch on STATUS_REG & 0x08 in the debugger

B. Add a watch on STATUS_REG and manually check bit 3 in code

C. Modify the code to print bit 3 value every cycle

D. Declare a new variable to store bit 3 and watch that variable

Solution

1. Step 1: Understand watching specific bits

   Debuggers allow watching expressions like bitwise AND to monitor specific bits.

2. Step 2: Choose the correct watch expression

   Watching STATUS_REG & 0x08 isolates bit 3 for monitoring directly.

3. Step 3: Compare other options

   Watching whole register or printing in code is less direct; declaring variable adds overhead.

4. Final Answer:

   Add a watch on STATUS_REG & 0x08 in the debugger -> Option A

5. Quick Check:

   Watch bit with bitwise AND expression [OK]

Hint: Use bitwise AND in watch to isolate specific bits [OK]

Common Mistakes:

• Watching whole register without isolating bit
• Trying to watch bits only via code prints
• Declaring extra variables unnecessarily