What Are AND / OR / NOT Gates?
AND, OR and NOT are the three basic logic gates from which all digital circuits are built. This article covers the gates from the IC side; for the general concept see the what are logic gates article.
AND Gate
Its output is 1 only when all inputs are 1. It's the "if both are true" logic. Its classic IC is the 7408 (quad 2-input AND).
OR Gate
Its output is 1 when at least one input is 1. It's the "if any is true" logic. A common IC is the 7432.
NOT Gate
It inverts its single input: makes 1 a 0 and 0 a 1 (inverter). Its IC is the 7404 (hex inverter).
| A | B | AND | OR |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 |
| 1 | 1 | 1 | 1 |
Derived Gates and Circuits
From these three basic gates the NAND/NOR/XOR gates derive; and from their combinations more complex combinational circuits such as the decoder are built.
Logic Families (TTL / CMOS)
The same gate is made in different families: 74LS (TTL), 74HC/HCT (CMOS). The family sets the supply voltage, speed and power consumption; the logic levels must be chosen compatibly.
Common Mistakes
- Floating input: an unused gate input is undefined; a pull-up/pull-down is essential.
- Mixing families: connecting TTL and CMOS levels directly causes problems.
- Current limit: drawing too much current from a gate output corrupts the level.
Frequently Asked Questions
What is the 74 series?+
Are these gates also inside an MCU?+
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