Digital comparator

For other uses, see comparator (disambiguation).

A digital comparator or magnitude comparator is a hardware electronic device that takes two numbers as input in binary form and determines whether one number is greater than, less than or equal to the other number. Comparators are used in a central processing units (CPU) and microcontrollers. Examples of digital comparator include the CMOS 4063 and 4585 and the TTL 7485 and 74682-'89.

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

Comparator truth tables

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

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

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

Inputs				Outputs
A1	A0	B1	B0	A < B	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

Implementation

Consider two 4-bit binary numbers A and B such that

A = A₃A₂A₁A₀

B = B₃B₂B₁B₀

Here each subscript represents one of the digits in the numbers.

Equality

The binary numbers A and B will be equal if all the pairs of significant digits of both numbers are equal, i.e.,

A₃ = B₃, A₂ = B₂, A₁ = B₁ and A₀ = B₀

Since the numbers are binary, the digits are either 0 or 1 and the boolean function for equality of any two digits A_i and B_i can be expressed as

$x_i= A_i \cdot B_i + \overline{A}_i \cdot \overline{B}_i$.

x_i is 1 only if A_i and B_i are equal.

For the equality of A and B, all x_i variables (for i=0,1,2,3) must be 1.

So the quality condition of A and B can be implemented using the AND operation as

(A = B) = x₃x₂x₁x₀

The binary variable (A=B) is 1 only if all pairs of digits of the two numbers are equal.

Inequality

In order to manually determine the greater of two binary numbers, we inspect the relative magnitudes of pairs of significant digits, starting from the most significant bit, gradually proceeding towards lower significant bits until an inequality is found. When an inequality is found, if the corresponding bit of A is 1 and that of B is 0 then we conclude that A>B.

This sequential comparison can be expressed logically as:

$(A>B)=A_3 \cdot \overline{B}_3+x_3 A_2 \overline{B}_2+x_3 x_2 A_1 \overline{B}_1+x_3x_2x_1 A_0 \overline{B}_0$

$(A<B)=\overline{A}_3 \cdot B_3+x_3 \overline{A}_2 B_2+x_3 x_2 \overline{A}_1 B_1+x_3x_2x_1 \overline{A}_0 B_0$

(A>B) and (A < B) are output binary variables, which are equal to 1 when A>B or A<B respectively.

See also

• 4000 series
• 7400 series
• Sorting network

External links

• Digital Comparators by Texas Instruments

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