Bits and registers in C

Every peripheral is controlled through registers: memory locations in which each bit has a meaning. Bit 5 of one register might enable a pin; bit 3 of another might signal that a byte has arrived. For this reason, embedded C often changes one bit and leaves the others unchanged. There are exactly 4 idioms for this, and they are worth learning by heart.

First, make a mask with a 1 in the required position: (1u << n). The u makes the constant unsigned, which avoids problems when n is 31. Set a bit with OR: reg |= (1u << 3). Clear it with AND NOT: reg &= ~(1u << 3). Toggle it with XOR: reg ^= (1u << 3). Test it with AND: if (reg & (1u << 3)) is true when bit 3 is set.

The compound forms |=, &= and ^= read the register, change one bit and write it back, so the other bits keep their values. Plain assignment (reg = 0x08) overwrites all the other bits, which is a common bug.

Hexadecimal notation shows bits clearly. Each hex digit is 4 bits: 0x0F is 0000 1111 and 0x28 is 0010 1000, so in 0x28 bits 5 and 3 are set. When in doubt, write the number out in binary.

Some registers hold a small number in a few bits, for example a 3-bit field in bits 4 to 6. To read it, shift the field down to bit 0, then mask off everything above it: (r >> 4) & 0x7. To write it, clear the field first, then OR in the new value shifted into place.