Building D Flip Flops

1
Building D Flip Flops

• Combinational Circuit Components
• Switches
• Voltage inverters
• D Clocked Latch
• Feedback to store bits
• D Flip Flop
• Two D clocked latches

2
Building D Flip Flops
D Flip Flops
D Clocked Latches
Primitive Combinational Circuits(Switches/Transis
ters)
3
Combinational Circuit Components
Control L
Control H
Normally Open Switch
open
closed
L
H
Normally Closed Switch
open
closed
L
H
4
Combinational Circuit Components
Voltage Inverters (2 symbols)
Active Device
L
H
L
H
5
A Simple 1-Bit Memory
Q
D
State value (1 bit)
Input to load new state value
6
A Simple 1-Bit Memory
Q
D
State value (1 bit)
Input to load new state value
Two configurations Hold (store) hold onto the
state value Load load a new state value
7
Holding (Storing)With Voltage Inverters
Devices drive each other
8
Holding (Storing)With Voltage Inverters
Devices drive each other
H
9
Holding (Storing)With Voltage Inverters
Devices drive each other
H
H
L
10
Holding (Storing)With Voltage Inverters
Devices drive each other
H
H
L
L
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Holding (Storing)With Voltage Inverters
Devices drive each other
H
H
L
L
12
Simple Memory Two Configurations
Q
D
State value (1 bit)
Input to load new state value
Two configurations Hold (store) hold onto the
state value Load load a new state value
13
Simple Memory Two Configurations
Hold
Load
D
Q
D
Q
14
Simple Memory Two Configurations
Hold
Load
D
Q
D
Q
Switches
Switches
15
D Clocked Latch
clock
Q
D
clock
clock L Hold clock H Load
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D Clocked Latch
Hold
Q
D
Q
Clock
L
Clock
Q D
Load
It similar to a D flip flop but it reacts to the
clock differently
D
Q
Clock H
Its transparent
17
Comparing Flip Flop and Latch
Q
D
Clock
Clock
D
Q-FlipFlop
Q-Latch
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Comparing Flip Flop and Latch
Q
D
Clock
Clock
D
T1
T2
Q-FlipFlop
T3
Q-Latch
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Comparing Flip Flop and Latch
Q
D
Clock
Clock
D
Q-FlipFlop
Q-Latch
20
Comparing Flip Flop and Latch
Q
D
Clock
Clock
D
Q-FlipFlop
Q-Latch
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D Flip Flop vs. D Clocked Latch

• D flip flop
• Triggered on positive edge of clock
• Output Q (and state) changes only at a time
  instant
• D clocked latch
• Output Q changes (with D) while clock is H
• Output Q changes during a window of time
• Trickier to use since lots of changes can happen
  during a time duration
• Flip flops are preferred to latches in designing
  circuits
• Latches are used in memory circuits, e.g., RAM

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D Flip Flop
D
Q
D
Q
Clock
Clock
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D Flip Flop
D
Q
D
Q
Clock
Clock
Load D Q
Clock L
Hold
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D Flip Flop
D
Q
D
Q
Clock
Clock
Load D Q
Clock L
Hold
Load D Q
Clock H
Hold
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D Flip Flop
Load (loads input)
Hold (output doesnt change)
L
D
Q
D
Q
Clock
Clock
Input really doesnt get stored until the
upward clock transition
Clock
26
D Flip Flop
Load (loads input)
Hold (output doesnt change)
L
H
H
D
Q
D
Q
Clock
Clock
Input really doesnt get stored until the
upward clock transition
Clock
27
D Flip Flop
Load (loads input)
Load (transfers state to output)
Hold (output doesnt change)
Hold
H
H
L
H
H
D
Q
D
Q
Q
D
D
Q
Clock
Clock
Clock
Clock
Input really doesnt get stored until the
upward clock transition
Clock
28
Summary

• Combinational circuit components
• switches and voltage inverters
• D clocked latch
• Built from switches and voltage inverters
• 2 configurations load and hold
• D flip flop
• Built from two D latches in series