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

Recall & Review

beginner

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

Setting a specific bit means changing that bit's value to 1 while leaving other bits unchanged.

beginner

Which operator is commonly used in C to set a specific bit in a register?

The bitwise OR operator (|) is used to set a specific bit.

intermediate

Explain the expression: register |= (1 << n);

It shifts 1 to the left by n positions to create a mask with only bit n set, then ORs it with the register to set bit n.

intermediate

Why is it important to use bitwise OR instead of assignment when setting a bit?

Using OR preserves other bits in the register; assignment would overwrite all bits, losing existing data.

beginner

Write a C code snippet to set bit 3 of a register named REG.

REG |= (1 << 3);

What does the expression (1 << 5) do in C?

A Sets bit 5 to 1

B Shifts the number 1 left by 5 bits

C Clears bit 5

D Performs bitwise AND with 5

The << operator shifts the number 1 left by 5 bits, creating a mask with bit 5 set.

Which operator is used to set a bit without changing other bits?

A | (OR)

B ^ (XOR)

C & (AND)

D ~ (NOT)

The OR operator sets bits to 1 where the mask has 1s, preserving other bits.

What happens if you use assignment (=) instead of OR (|=) to set a bit?

A Only the targeted bit is set

B All bits are cleared

C The entire register is overwritten, losing other bits

D Nothing changes

Assignment replaces the whole register value, so other bits are lost.

How do you set bit 0 of a register named REG?

A REG &= ~(1 << 0);

B REG |= (1 >> 0);

C REG ^= (1 << 0);

D REG |= (1 << 0);

Shifting 1 by 0 leaves it as 1, so OR sets bit 0.

What is the result of REG |= (1 << n); if bit n was already set?

A Bit n remains set

B Register value becomes zero

C All bits are cleared

D Bit n is cleared

OR with 1 keeps the bit set; no change if already set.

Describe how to set a specific bit in a register using C code.

Think about creating a mask with 1 shifted to the bit position.

Explain why using |= is better than = when setting a bit in a register.

Consider what happens to bits not targeted.