Page 145 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

126 H Chapter Eleven

1 --
-clear
0 ---I
L
1 _--- _---_- I
serial-in I I
0- I I I I
1
clack
0
I I I I
I I I I I
I I I I I
9PI I I I I I I
0 I I I I I
1
q[ll x XI I I I I I I I I
0
1
qC21 x XI I I I I I I
0
1 I
qc31 XXI I
0 I
-
Time
Figure 11-20. Waveform for SIP0 shift register

Similarly, when the next active edge occurs on the clock input, the 5eria I-in
so
input is still 0, this is the value that’s loaded into the first flip-flop. At the
same time, the O value from the first flip-flop is loaded into the second, the I
value from the second flip-flop is loaded into the third, and the O value from
the third flip-flop is loaded into the fourth. And so it goes. . .
Other common shift register variants are the paralEel-in-serial-out (PISO) ,

and the serial-in-serial-out (SISO); for example, consider a 4-bit SISO shift
register (Figure 11-21).

Counters

Counter functions are also commonly used in digital systems. The number
of states that the counter will sequence through before returning to its original
value is called the modulus of the counter. For example, a function that counts
from 0000, to 11 1 1, in binary (or 0 to 15 in decimal) has a modulus of sixteen
and would be called a modulo-16, or mod-16, counter. Consider a modulo-16
counter implemented using D-type flip-flops (Figure 11-22).