Hardware Reference
In-Depth Information
A
B
A
B
XOR
0
0
0
0
1
1
A
1
0
1
B
1
1
0
(a)
(b)
A
B
A
B
A
A
B
B
(c)
(d)
Figure 3-8.
(a) The truth table for the
XOR
function. (b)-(d) Three circuits for
computing it.
adopt the convention that 0 volts is logical 0 and 1.5 volts is logical 1, called
posi-
tive logic
, we get the truth table of Fig. 3-9(b), the AND function. If, however, we
adopt
negative logic
, which has 0 volts as logical 1 and 1.5 volts as logical 0, we
get the truth table of Fig. 3-9(c), the OR function.
A
B
F
0
V
0
V
0
V
5
V
A
B
F
A
B
F
0
V
0
V
0
0
0
0
1
1
1
1
0
V
5
V
0
1
1
0
5
V
0
V
1
0
0
1
0
1
1
0
5
V
5
V
1
1
0
0
(a)
(b)
(c)
Figure 3-9.
(a) Electrical characteristics of a device. (b) Positive logic. (c) Neg-
ative logic.
Thus, the convention chosen to map voltages onto logical values is critical.
Except where otherwise specified, we will henceforth use positive logic, so the
terms logical 1, true, and high are synonyms, as are logical 0, false, and low.
