Setting a specific bit in a register in Embedded C

Introduction

We set a specific bit in a register to turn on a feature or control hardware without changing other settings.

Turning on an LED connected to a microcontroller pin.

Enabling a sensor by setting its control bit.

Configuring communication settings by setting bits in a control register.

Activating interrupts by setting the interrupt enable bit.

Changing device modes by setting mode bits in a register.

Syntax

Embedded C

REGISTER |= (1 << BIT_POSITION);

REGISTER is the name of the hardware register.

BIT_POSITION is the zero-based position of the bit to set (0 for least significant bit).

Examples

Sets bit 3 of PORTA register to 1, turning on the connected pin.

Embedded C

PORTA |= (1 << 3);

Sets bit 0 of CONTROL_REG to enable a feature.

Embedded C

CONTROL_REG |= (1 << 0);

Sets the highest bit (bit 7) of STATUS register.

Embedded C

STATUS |= (1 << 7);

Sample Program

This program starts with all bits off in registerA. It sets bit 2 to 1 using the bitwise OR and shift. Then it prints the register bits from left to right.

Embedded C

#include <stdio.h>

int main() {
    unsigned char registerA = 0b00000000; // all bits off
    int bit_to_set = 2; // we want to set bit 2

    // Set bit 2
    registerA |= (1 << bit_to_set);

    printf("Register after setting bit %d: 0b", bit_to_set);
    for (int i = 7; i >= 0; i--) {
        printf("%d", (registerA >> i) & 1);
    }
    printf("\n");

    return 0;
}

Important Notes

Using |= keeps other bits unchanged while setting the chosen bit.

Bit positions start at 0 from the right (least significant bit).

Make sure the register variable is wide enough to hold the bits you want to set.

Summary

Use REGISTER |= (1 << BIT_POSITION); to set a specific bit.

This changes only the chosen bit without affecting others.

Bit positions count from 0 at the rightmost bit.