Digital Systems: Principles and Design

Implementation of Combinational

Logic by Standard ICs and

Programmable ROM Memories

We learnt in earlier chapters that a combinational circuit has following characteristics:

1. These have n inputs and m outputs. For examples, (i) A NOT (inverter) gate, n =

1 and m = 1. (ii) An XOR gate, n = 2 and m = 2. (iii) An 8-bit adder, which accepts

in its inputs a carry bit, 8 bits of X and 8 bits of Y and results at outputs the 8 bit

sum Z and final carry CY, n = 17 and m = 9.

2. Logic state, at any of the outputs in the combinational circuits, depends only on

the inputs at any given instant (not considering always present propagation de-

lay period) and is