Building D Flip Flops

1
Building D Flip Flops

• Combinational circuit components
• D Clocked Latch similar to a flip flop but
  simpler
• D flip flop

2
Combinational Circuit Components
Normally Closed Switch
Voltage Inverters
closed
L
open
H
3
D clocked latch
Hold
Q
L
Clock
Load
It similar to a D flip flop but it reacts to the
clock differently
D
Q
Q D
Clock H
Its transparent
4
Differences between a D flip flop and a 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 duration of time
• Trickier to use since lots of changes can happen
  during a time duration

5
Building a D clocked latch
Hold Clock L
Devices drive each other
Q
state stored here
Two configurations that depend on the clock
D
Load Clock H
Q
6
Building a D clocked latch
clock L Hold clock H Load
7
Building a D flip flop
D
Q
D
Q
Clock
Clock
Load D--gtQ
Hold
Clock L
Load D--gtQ
Hold
Clock H
8
Building a D flip flop
Load (loads input)
Load (transfers state to output)
Hold (output doesnt change)
Hold
Input really doesnt get stored until the edge
Clock
9
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