Basics
Logic states
Digital circuits interpret voltage ranges as logic states. The two common states are called LOW and HIGH, or 0 and 1. Those names do not mean that every IC uses exactly 0 V and 5 V.
Supply pins
Digital ICs require power even when the schematic symbol emphasizes only logic inputs and outputs. Bipolar logic commonly uses labels such as VCC and GND. CMOS literature may use VDD and VSS. The exact pin names depend on family and manufacturer.
Pin 1 and orientation
Dual-inline packages commonly use a notch, dot, bevel, or molded mark to identify the end containing pin 1. Pin numbers then proceed counterclockwise when viewed from the top of the package.
Inputs and outputs
An input senses a logic state. An output drives a logic state. Some pins can be three-state outputs, bidirectional buses, open-collector/open-drain outputs, clock inputs, asynchronous controls, analog pins, or mode-select inputs.
Truth tables
A truth table lists every important input combination and the corresponding output state.
| A | B | AND | OR | XOR |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 1 | 1 |
| 1 | 1 | 1 | 1 | 0 |
Unused inputs
Unused digital inputs should normally be tied to a defined logic level by a method appropriate to the logic family. A floating input can pick up noise and switch unpredictably.
Floating CMOS inputs are especially troublesome because they can drift into the linear region, causing both complementary transistors to conduct at once and increasing power consumption.
Decoupling capacitors
A digital IC draws short bursts of current when internal transistors switch. A small ceramic bypass capacitor placed close to the supply pins provides local current and keeps supply wiring inductance from creating large voltage spikes.
A common starting point for ordinary logic is a small capacitor such as 0.1 µF per package, but actual high-speed designs should follow device and board requirements rather than one universal number.
Fan-out
Fan-out describes how many logic inputs one output can drive while maintaining valid voltage levels. In TTL it is strongly related to input and output current. In CMOS, DC fan-out can be very large, but each added input adds capacitance and therefore slows transitions.
Propagation delay
An output does not change instantly when an input changes. The delay between input and output is called propagation delay. Complex logic paths accumulate delay.
Absolute maximum versus recommended operation
Absolute maximum ratings are damage limits, not normal design values. Use the recommended operating conditions for supply voltage, input range, output current, temperature, and timing.