Digital Systems

Chapter 12

Implementation of Combinational Logic by Standard ICs and Programmable ROM Memories

OBJECTIVE

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 delay period) and is not correlated at all with any of its previous outputs or outputs.

3. A truth table ...