Logic Gates

Fundamental logic gates: AND, OR, NOT, NAND, NOR, XOR and their properties.

Introduction to Digital Logic

Digital logic operates on two levels: 0 (false, low, GND) and 1 (true, high, VCC).

A logic gate is an electronic circuit that performs a logical operation on one or more input bits to produce one output bit.

Real Logic Levels

A 0 and a 1 are not magic values: they are voltage ranges recognized by components.

In common positive logic:

• 0 is a low voltage, close to GND;
• 1 is a high voltage, close to VCC;
• between them, there is an uncertain region to avoid.

A component does not switch exactly at VCC / 2. It has input thresholds: below one level, the signal is read as 0; above another level, it is read as 1. The gap between the produced signal and the required threshold is the noise margin.

Noise, Load and Delays

A digital signal can be disturbed by noise, bouncing, coupling between traces or poor supply decoupling. Digital logic tolerates these imperfections better than analog circuits, but it does not make them disappear.

Three ideas matter: