Logic gates are the elemental electronic components of digital and computer circuits. Digital circuits have two valid voltage levels (high/true/1 and low/false/0). The inputs and outputs (I/O) of digital components can therefore be referred to as 1s and 0s instead of voltage values, where voltage ranges for the two levels are specified.
The most common family of logic integrated circuits (ICs), called TTL, are powered by a 5 V supply. Voltages above 2 V are a 1 and below 0.8 V are a 0. In between these voltages, the logic level is undefined (invalid). For the 4000 family of CMOS logic, a 1 is > 0.7
× VDD and a 0 is < 0.3× VDD, where VDD is the supply voltage (such as, 12 V).The simplest logic component is an inverter, shown below. The "bubble" at the output indicates inversion.
When its input is 0, its output is 1, and vice-versa. To specify the function of such logic devices, a truth table is used, showing all combinations of inputs and their outputs. For an inverter – the logic NOT function – the truth table is:
|
INPUT |
OUTPUT |
|
0 |
1 |
|
1 |
0 |
A basic logic gate is the AND gate, shown below:
The truth table for an AND gate is:
|
A |
B |
Y |
|
0 |
0 |
0 |
|
1 |
0 |
0 |
|
0 |
1 |
0 |
|
1 |
1 |
1 |
In other words, the output of an AND gate is high (1) only when all (both) inputs are high. The output is low when any (either) of the inputs is low.
An OR gate has a high output if any (either) input is high. The OR gate symbol is shown below.
The OR gate truth table is:
|
A |
B |
Y |
|
0 |
0 |
0 |
|
1 |
0 |
1 |
|
0 |
1 |
1 |
|
1 |
1 |
1 |
An OR gate has a low output only when all inputs are low.
When an inverter follows a gate it becomes a NOT AND (NAND) or NOT OR (NOR) gate. The symbols for these gates is the same as AND and OR gates but with an inverter "bubble" at their outputs, as shown below.
Commercial gates are available as ICs. Usually, several gates are available in one IC package. The 4001 contains 4 NOR gates, designated as A - D.
