CS 140L Lecture 4 FlipFlops, Shifters and Counters

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CS 140L Lecture 4Flip-Flops, Shifters and
Counters

• Professor CK Cheng
• CSE Dept.
• UC San Diego

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• F-F
• Shift register
• Counter (Asynchronous)
• Counter (Synchronous)

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Flip-Flops
DFF
D
Asynchronous Clear
Q
Inputs Output
CE
C
CLR CE D C Q 1 X X X 0
0 0 X X No change 0 1 1
1 0 1 0 0
CLR
Clock Enable
CLK 1
CLK 0
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DFF Timing
CLK
tsetup
thold
t
D
t
Q
t
tcq
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Input Timing Constraints

• Setup time tsetup time before the clock edge
  that data must be stable (i.e. not changing)
• Hold time thold time after the clock edge that
  data must be stable
• Aperture time ta time around clock edge that
  data must be stable (ta tsetup thold)

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Output Timing Constraints

• Propagation delay tpcq time after clock edge
  that the output Q is guaranteed to be stable
  (i.e., to stop changing)
• Contamination delay tccq time after clock edge
  that Q might be unstable (i.e., start changing)

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Setup Time Constraint

• The setup time constraint The maximum delay from
  register R1 through the combinational logic.
• The input to register R2 must be stable at least
  tsetup before the clock edge.

Tc tpcq tpd tsetup tpd Tc (tpcq
tsetup)
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Hold Time Constraint

• The hold time constraint depends on the minimum
  delay from register R1 through the combinational
  logic.
• The input to register R2 must be stable for at
  least thold after the clock edge.

thold lt tccq tcd tcd gt thold - tccq
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2) A 3 Bit Shift Register
B
D
C
A
Q
Q
Q
D
D
D
CLK
Time Steps A B C D

• 0 0 X X X
• 1 0 X X
• 2 0 1 0 X
• 1 0 1 0
• 1 1 0 1
• 0 1 1 0
• 0 0 1 1
• 1 0 0 1

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3) A 3 Bit Counter (Asynchronous)
B
A
C
Q
Q
Q
1
1
T
T
T
1
CLK
Assume A(0) B(0) C(0) 0
Time C B A

• 0 0 0 0 0
• 1 1 1 7
• 2 1 1 0 6
• 1 0 1 5
• 1 0 0 4
• 0 1 1 3
• 0 1 0 2
• 0 0 1 1

CLK
t
A
1
0
1
0
t
B
1
1
0 0
t
C
1 1 1 1
t
7 6 5 4