Logic Gates

Logic gates are circuits whose output depends on Boolean inputs (HIGH/1 or LOW/0). Building blocks of all digital systems.

Basic gates: AND, OR, NOT. Universal gates: NAND, NOR (any logic can be built from either alone).

Truth tables define each gate's behaviour. AND: output 1 only if all inputs 1. OR: output 1 if any input 1. NOT: inverts input.

XOR (exclusive OR): output 1 when inputs differ. XNOR: complement of XOR.

DeMorgan's laws: (A·B)' = A' + B' and (A+B)' = A'·B'. Lets you convert AND into OR (with inversions) and vice versa.

CMOS technology builds all logic from complementary p-MOS and n-MOS pairs — very low static power consumption.

Standard CMOS voltages: V_DD = 3.3 V or 5 V; logic 0 < ~0.8 V, logic 1 > ~2.0 V (TTL levels).

Combinational logic: output depends only on current inputs (adders, multiplexers). Sequential logic: output also depends on past (flip-flops, registers, memory).