Digital comparator

Digital comparator

A digital comparator is a hardware electronic device that compares two numbers in binary form and generates a one or a zero at its output depending on whether they are the same or not.

Comparators can be used in a central processing unit (CPU) or microcontroller in branching software. A comparator can be simulated by subtracting the two values (A & B) in question and checking if the result is zero. This works because if A = B then A - B = 0.

The analog equivalent is the comparator. Many microcontrollers have analog comparators on some of their inputs that can be read or trigger an interrupt.

The operation of a single bit digital comparator can be expressed as a truth table:

Inputs Outputs
A B A A=B A>B
0 0 0 1 0
0 1 1 0 0
1 0 0 0 1
1 1 0 1 0

The operation of a two bit digital comparator can be expressed as a truth table:

Inputs Outputs
A_1 A_0 B_1 B_0 A A=B A>B
0 0 0 0 0 1 0
0 0 0 1 1 0 0
0 0 1 0 1 0 0
0 0 1 1 1 0 0
0 1 0 0 0 0 1
0 1 0 1 0 1 0
0 1 1 0 1 0 0
0 1 1 1 1 0 0
1 0 0 0 0 0 1
1 0 0 1 0 0 1
1 0 1 0 0 1 0
1 0 1 1 1 0 0
1 1 0 0 0 0 1
1 1 0 1 0 0 1
1 1 1 0 0 0 1
1 1 1 1 0 1 0

Examples include the CMOS 4063 and 4585 and the TTL 7485 and 74682-'89.

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