1. CS 140 Lecture 13
Professor CK Cheng
11/12/02

2. Part II. Standard Modules
Decoder, Encoder
Multiplexer, DeMultiplexer

3. MUX (Multiplexer) EN If EN = 1 D2n-1,0 y = Di i = (Sn-1, .. , S0) y
(Data
input)
(Control
Input)
Sn-1,0

4. Example EN D0 D1 1 y If D0 = 0 and S1S0 = 00, y = 0D1 1 y
If D0 = 0 and S1S0 = 00, y = 0
If D0 = 1 and S1S0 = 00, y = 1 D2 2 D3 3 S1 S0

5. Multiplexers as a universal set
Example: Given f (a,b,c) = Sm (0,1,7) + Sd(2)
1) Implement with an 8-input Mux. EN
Id a b c f 1 1 2 3 4 5 6 7 1 y 1 S2 S1 S0 a b c

6. 2) Implement with 4-input Muxesa 1 b 1
c = 0 1 -
c = 1 1
D (c)
D0 (c) =1
D1 (c) =0
D2 (c) =0
D3 (c) =c 1 1 y 2 c 3 S1 S0
y(a,b,c)
y(0,0,c) = D0
y(0,0,1) = D1
y(1,0,c) = D2
y(1,1,c) = D3 a b

7. 2) Implement with 2-input Muxesa 1
D (b,c)
D0 (b,c)
D1 (b,c) EN b’ EN
D0 (b,c) = b’
D1 (b,c) = bc y 1 1 1 b b c a c c
D1 (b,c) b b 1
c = 0
c = 1 1
l1(0) = 0
l1(c) = c

8. Demultiplexers EN yi = x if i = (Sn-1, .. , S0) & EN = 1 = 0 otherwise
y2n-1
(Control
Input)
Sn-1

9. Shifter xn xn-1 x0 x-1 yi = xi-1 if EN = 1, s = 1, and d = left
= xi+1 if EN = 1, s = 1, and d = right
= xi if EN = 1, s = 0
= 0 if EN = 0 s
s / n EN d
l / r
yn-1 y0
xi+1 xi
xi-1
Can be implemented with a mux s 1 EN d yi

10. Barrel Shifter shift x 0 1 0 1 0 1 O or 1 shift s0 O or 2 shift s1 0 1
0 1
0 1
0 1
O or 1 shift s0
O or 2 shift s1
0 1
0 1
0 1
0 1
0 1
O or 4 shift s2 y
0 1
0 1
0 1
0 1
0 1
0 1