Digital Design Fundamentals·Signals, Bits & Logic
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A logic gate takes one or more bits in and produces one bit out, following a fixed rule. Three gates are the foundation:
Underneath, each gate is a few transistors: real switches in silicon. AND is switches in series, OR is switches in parallel, NOT flips its input.
Figure: The three core gate symbols drawn large and labelled: the flat-backed D-shaped AND gate, the curved shield-shaped OR gate
A logic gate takes one or more bits in and produces one bit out, following a fixed rule. Three gates are the foundation:
Underneath, each gate is a few transistors: real switches in silicon. AND is switches in series, OR is switches in parallel, NOT flips its input.
The circuit above feeds the same two inputs, a and b, into an AND, an OR and a NOT gate at once.
Try it:
Every combination of a and b gives one fixed set of outputs. There is no memory and no history here, only the rule.
A truth table pins a gate down completely: every possible input, and the output for each.
NOT:
| A | Y |
|---|---|
| 0 | 1 |
| 1 | 0 |
AND and OR:
| A | B | AND | OR |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 |
Two inputs give four rows. If two circuits share a truth table, they do the same logic, whatever they look like inside.
These three gates map straight onto plain English:
Almost every digital circuit is built by wiring gates like these together. Next lesson we meet three more gates, including one that can build all the others by itself.
