文档介绍：Flip-flops
Chapter4
Simple counter
Counting is a fundamental function of digital circuits.
A digital counter consists of a collection of flip-flops that change states (set or reset) in a prescribed sequence.
Asynchronous counter
The output of one flip-flop provides the clock for the next flip-flops.
When all of the flip-flop outputs are available, these are called modulo-n counters where n represent the terminal counter.
Divide by 2,4 and 8 Counters
K
J
Q1’
Q1
C
CLK
K
J
Q2’
Q2
C
1
1
VCC
CLK
Q1
Q2
0
0
1
0
0
1
1
1
0
0
1
0
Johnson Counter (synchronous)
The output of one flip-flop directly connects to the input of another.
D
Q1’
Q1
C
D
Q2’
Q2
C
D
Q3’
Q3
C
D
Q4’
Q4
C
CLK
1
1
0
0
D
Q1’
Q1
C
D
Q2’
Q2
C
D
Q3’
Q3
C
D
Q4’
Q4
C
CLK
CLK
Q1
Q3
Q4
Q2
t
t’
1 2 3 4 5 6 7 8 9
Time t, CLK=1, Q1=1, Q2=1, Q3=0, Q4=0
Time t’, CLK=0, Q1=Q4’t=1, Q2=Q1t=1, Q3=Q2t=1, Q4=Q3t=0
1
1
1
0
Johnson Counter (synchronous)
The Johnson counter can produce a series of outputs from each flip-flop that is offset by a clock pulse from the preceding flip-flop output.
The Johnson counter can be used to produce time delay; the output of each flip-flop is delayed by one clock pulse from the previous output.
Ring Counter (synchronous)
The output of one flip-flop directly connects to the input of another.
The ability to load the flip-flops to a