Example: Modulo-4 Counter

A-73
Appendix A: Digital Logic
PLA Vending Machine Controller
x 1
x 0
(a)
Base 10
equivalent
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Present
state Coin
s 1 s 0 x 1 x 0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
5 × 5
PLA
D
Q s0
D
Q s1
0 0
1 1
0 0
1 d
0 1
1 1
0 0
1 d
0 1
1 0
0 0
1 d
0 0
1 0
0 0
1 d
(b)
Dispense
Next Return nickel
state Return dime
s 1 s 0 z 2 z 1 z 0
1
0
0
d
0
1
0
d
1
0
0
d
0
0
1
d
0
0
1
d
0
0
1
d
0
1
1
d
1
1
1
d
0
0
1
d
0
0
0
d
0
0
1
d
0
1
1
d
0
0
0
d
0
0
1
d
0
0
1
d
0
0
1
d
z 2
z 1
z 0
CLK
s 1 s 0 x 1 x 0
s 1 s 0 z 2 z 1 z 0
Principles of Computer Architecture by M. Murdocca and V. Heuring © 1999 M. Murdocca and V. Heuring
(c)
0
1
2
4
5
6
8
9
10
12
13
14