ECE 301 – Digital Electronics
Latches and Flip-Flops
(Lecture #18)
The slides included herein were taken from the materials accompanying
Fundamentals of Logic Design, 6th Edition, by Roth and Kinney,
and were used with permission from Cengage Learning.
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Brief introductionto
Sequential Logic Circuits
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Sequential Logic Circuits
● The output of a sequential logic circuit is dependent not only on the present inputs, but also on the past sequence of the inputs.
● A sequential logic circuit must “remember” the past history of the inputs.
● It does this using basic memory elements.
– Latches
– Flip-Flops
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Sequential Logic Circuits
Combinational
Logic
CircuitMemory
inputsoutputs
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Basic Memory Elements
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Basic Memory Elements
● Latch
– Clock input is level sensitive.
– Output can change multiple times during a clock cycle.
– Output changes while clock is active.
● Flip-Flop
– Clock input is edge sensitive.
– Output can change only once during a clock cycle.
– Output changes on clock transition.
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Basic Memory Elements
Both latches and flip-flops use feedback to achieve “memory”.
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Feedback Circuit with 2 Stable States
What is the problem with this circuit?
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Latches
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Set-Reset (SR) Latch
● A Set-Reset Latch has two inputs
– Set (S) input
– Reset (R) input
● It can be constructed from two cross-coupled NOR gates or two cross-coupled NAND gates.
● It has three modes of operation
– Set: Latch output set to 1 (Q+ = 1)
– Reset: Latch output reset to 0 (Q+ = 0)
– Store: Latch output does not change (Q+ = Q)
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SR Latch: using NOR gates
A B NOR
0 X X'
1 X 0
Feedback NOR gates
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SR Latch: Set (S = 1, R = 0)
A B NOR
0 X X'
1 X 0
1
01
0
P = Q'
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SR Latch: Reset (S = 0, R = 1)
A B NOR
0 X X'
1 X 0
0
10
1
P = Q'
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SR Latch: Store (S = 0, R = 0)
A B NOR
0 X X'
1 X 0
0
01
0
P = Q'
Initial Condition: P = 0, Q = 1
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SR Latch: Store (S = 0, R = 0)
A B NOR
0 X X'
1 X 0
0
00
1
P = Q'
Initial Condition: P = 1, Q = 0
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• If S = 1 (Set), Q+ = 1
• If R = 1 (Reset), Q+ = 0
• If S = R = 0, Q+ = Q (no change)
• S = R = 1 is not allowed.
SR Latch: Behavior
Nextvalue
Presentvalue
S R Q Q+
0 0 0 0
0 0 1 1
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 not
1 1 1 allowed
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P ≠ Q′
SR Latch: Improper Operation
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SR Latch: Symbol
Q'
Q
SRLatch
S
R
Q
Q'
always
complementary
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SR Latch: Timing Diagram
store set store reset
ε = propagation delay of the latch
Q'
Q
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SR Latch: Characteristic Equation
Characteristic Equation: Q+ = S + R'.Q (S.R = 0)
Q = present state
Q+ = next state
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SR Latch: using NAND gates
A B NAND
0 X 1
1 X X'
S' R' Q Q+
1 1 0 0
1 1 1 1
1 0 0 0
1 0 1 0
0 1 0 1
0 1 1 1
0 0 0 not
0 0 1 allowed
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Gated D Latch
● A Gated D Latch has two inputs
– Gate (G) input
– Data (D) input
● It can be constructed from an SR Latch and additional logic gates.
● It has the following behavior
– G = 1: D is passed to Q (Q+ = D)
– G = 0: Q remains unchanged (Q+ = Q)
● Also referred to as a transparent latch.
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Gated D Latch: Circuit and Timing
NOR gates
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Gated D Latch: Symbol and Truth Table
No invalid inputs!
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Gated D Latch: Characteristic Equation
Characteristic Equation: Q+ = G'.Q + G.D
0
1
2
3
6
7
4
5
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Gated D Latch: using NAND gates
S'
R'
NAND gates
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Flip-Flops
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D Flip-Flop
● A D Flip-Flop has two inputs
– Clock (Ck) --- denoted by the small arrowhead
– Data (D)
● The output of the D Flip-Flop changes in response to the clock input only.
– not in response to a change in the D input
● The D Flip-Flop is edge-triggered not level-sensitive
– Positive (or rising) edge-triggered: 0 → 1
– Negative (or falling) edge-triggered: 1 → 0
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Q+ = D
Characteristic Equation:
D Flip-Flop
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D Flip-Flop: Timing Diagram
Which clock edge is the D flip-flop triggered on?
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D Flip-Flop (master-slave)
Gated D Latches
Enabled on opposite levels of the clock
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D Flip-Flop: Timing Diagram
Which clock edge is the D flip-flop triggered on?
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D Flip-Flop: Setup and Hold Times
Setup time Hold time
Propagation delay
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D Flip-Flop: Minimum Clock Period
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Questions?
