Page 337 - Embedded Microprocessor Systems Real World Design

Logic Table Using
Normal Logic Table Don’t Care

A B output A B output
0 0 0 0 X 0
0 1 0 X 0 0
1 0 0 1 1 1
1 1 1

X = don’t care

You can see that the logic table is the same for both cases. As long as A is low, the output
is low, regardless of what state B is in. Similarly, as long as B is low, the output will be low,
regardless of A. What this illustrates is an inhibit capability-if input A is a signal that
constantly switches between high and low, we can control whether the signal appears at the
output by controlling input B. While B is high, the output follows A. While B is low, the
output is low.
A similar don’t care table can be created for the OR gate:

Logic Table Using
Normal Logic Table Don’t Care
A B Output A B OUtpUt

0 0 0 0 0 0
0 1 1 X 1 1
1 0 1 1 X 1
1 1 1

X = don’t care

In this case, holding B high forces the output high, and taking B low allows the output
to follow A. All we did here was call 0 false and 1 true. These tables are the same as the
original logic tables for the AND and OR functions.

Negative Logic
Logic functions such as AND, NAND, and OR also can be used in an inverting configura-
tion, where a true is 0 and false is 1. This typically is indicated with inversion bubbles at the
input and output, as shown in Figure C.l. The logic of the invert-AND gate would be like
this:
If A is low AND B is low, the output is low.
If either A OR B is high, the output is high.

318 Appendix C