Clearing a specific bit in a register in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to clear a specific bit in a register?

Clearing a bit means setting that bit to 0 while leaving other bits unchanged.

beginner

Which bitwise operator is used to clear a specific bit in a register?

The bitwise AND operator (&) is used with the complement of a mask to clear a specific bit.

intermediate

Given a register variable 'reg' and a bit position 'pos', write the expression to clear that bit.

reg = reg & ~(1U << pos); // This clears the bit at position 'pos' in 'reg'.

intermediate

Why do we use the complement operator (~) when clearing a bit?

The complement operator creates a mask with all bits set to 1 except the bit to clear, which is 0. ANDing with this mask clears only that bit.

beginner

What happens if you clear a bit that is already 0?

Nothing changes; the bit remains 0 and other bits are unaffected.

Which operator is used to clear a specific bit in a register?

A Bitwise AND (&)

B Bitwise OR (|)

C Bitwise XOR (^)

D Bitwise NOT (~)

Clearing a bit uses AND with the complement mask to set that bit to 0.

What does the expression (1 << pos) do?

A Creates a mask with bit 'pos' set to 1

B Clears bit 'pos'

C Shifts all bits right by 'pos'

D Sets all bits to 1

It shifts 1 left by 'pos' places, creating a mask with only that bit set.

What is the purpose of using ~ (bitwise NOT) in clearing a bit?

A To shift bits left

B To set all bits to 1

C To invert the mask so the target bit is 0 and others are 1

D To toggle the bit

The NOT operator flips bits so the target bit becomes 0, allowing AND to clear it.

If reg = 0b1111 and you clear bit 2, what is the new value of reg?

A 0b1110

B 0b0111

C 0b1101

D 0b1011

Clearing bit 2 (counting from 0) changes 0b1111 to 0b1011.

What happens if you clear a bit that is already 0?

A The bit becomes 1

B The bit stays 0, no change

C All bits are cleared

D The register resets

Clearing a bit that is already 0 does not change the register.

Explain how to clear a specific bit in a register using bitwise operations.

Think about how to keep all bits except the one you want to clear.

Write a C code snippet to clear bit 5 of a variable named 'status'.

Use shift and complement operators with AND.

Practice

1. What does the expression register &= ~(1 << 3); do in embedded C?

easy

A. Sets the 3rd bit of the register to 1

B. Clears the 3rd bit of the register (sets it to 0) without changing other bits

C. Toggles the 3rd bit of the register

D. Clears all bits of the register

Solution

1. Step 1: Understand bit shifting and negation

   The expression 1 << 3 shifts 1 to the left by 3 bits, creating a mask with only the 3rd bit set (00001000).

2. Step 2: Apply bitwise NOT and AND assignment

   The bitwise NOT ~(1 << 3) flips all bits, making the 3rd bit 0 and others 1. ANDing this with the register clears only the 3rd bit.

3. Final Answer:

   Clears the 3rd bit of the register (sets it to 0) without changing other bits -> Option B

4. Quick Check:

   Clear bit 3 = register &= ~(1 << 3) [OK]

Hint: Use AND with NOT mask to clear a bit [OK]

Common Mistakes:

• Confusing clearing with setting a bit
• Using OR instead of AND
• Not applying bitwise NOT before AND

2. Which of the following is the correct syntax to clear bit 5 of a variable named reg?

easy

A. reg &= ~(1 << 5);

B. reg |= ~(1 << 5);

C. reg = reg & (1 << 5);

D. reg = reg | (1 << 5);

Solution

1. Step 1: Identify the operation to clear a bit

   Clearing a bit means setting it to 0. This is done by ANDing with the negation of the bit mask.

2. Step 2: Check each option's operation

   reg &= ~(1 << 5); uses &= ~(1 << 5), which correctly clears bit 5. Others either set or incorrectly manipulate bits.

3. Final Answer:

   reg &= ~(1 << 5); -> Option A

4. Quick Check:

   Clear bit 5 = reg &= ~(1 << 5) [OK]

Hint: Clear bit with AND and NOT mask [OK]

Common Mistakes:

• Using OR (|) instead of AND (&)
• Forgetting to negate the mask
• Assigning instead of using compound assignment

3. Given unsigned char reg = 0b11111111; and the operation reg &= ~(1 << 2);, what is the binary value of reg after execution?

medium

A. 0b11111011

B. 0b11111101

C. 0b11110111

D. 0b11111110

Solution

1. Step 1: Identify the bit to clear

   Bit 2 means the third bit from the right (0-based index). The mask is 1 << 2 = 0b00000100.

2. Step 2: Apply clearing operation

   Negate mask: ~0b00000100 = 0b11111011. AND with reg: 0b11111111 & 0b11111011 = 0b11111011.

3. Final Answer:

   0b11111011 -> Option A

4. Quick Check:

   Clear bit 2 from 0xFF = 0xFB (binary 11111011) [OK]

Hint: Clear bit 2 flips that bit to 0 only [OK]

Common Mistakes:

• Clearing wrong bit index
• Not negating the mask
• Confusing bit positions (1-based vs 0-based)

4. What is wrong with this code snippet if the goal is to clear bit 4 of reg?

reg = reg & (1 << 4);

medium

A. It causes a syntax error

B. It correctly clears bit 4

C. It sets all bits except bit 4 to 0, not clearing bit 4

D. It toggles bit 4 instead of clearing

Solution

1. Step 1: Analyze the operation performed

   The expression reg & (1 << 4) keeps only bit 4 and clears all others, effectively zeroing all bits except bit 4.

2. Step 2: Compare with correct clearing method

   To clear bit 4, we must AND with the negation of the mask: reg &= ~(1 << 4);. The given code does not do this.

3. Final Answer:

   It sets all bits except bit 4 to 0, not clearing bit 4 -> Option C

4. Quick Check:

   AND with mask keeps only that bit [OK]

Hint: AND with mask keeps only that bit, not clears it [OK]

Common Mistakes:

• Using AND with mask instead of negated mask
• Confusing clearing with masking
• Expecting toggling behavior

5. You have a 16-bit register uint16_t reg = 0xFFFF;. You want to clear bits 0, 4, and 15 simultaneously. Which code correctly does this?

hard

A. reg |= ( (1 << 0) | (1 << 4) | (1 << 15) );

B. reg &= ( (1 << 0) | (1 << 4) | (1 << 15) );

C. reg |= ~( (1 << 0) | (1 << 4) | (1 << 15) );

D. reg &= ~( (1 << 0) | (1 << 4) | (1 << 15) );

Solution

1. Step 1: Create a mask for bits to clear

   Bits 0, 4, and 15 correspond to masks 1<<0, 1<<4, and 1<<15. OR them to combine: (1<<0) | (1<<4) | (1<<15).

2. Step 2: Negate mask and AND with register

   Negate the combined mask to get zeros at bits 0,4,15 and ones elsewhere. AND with reg clears those bits.

3. Final Answer:

   reg &= ~( (1 << 0) | (1 << 4) | (1 << 15) ); -> Option D

4. Quick Check:

   Clear multiple bits by AND with negated OR mask [OK]

Hint: Combine bits with OR, negate, then AND to clear multiple bits [OK]

Common Mistakes:

• Using OR instead of AND to clear bits
• Not negating the combined mask
• Clearing only one bit instead of all three