Flip Flops

CS1104 – Computer Organization http://www.comp.nus.edu.sg/~cs1104 Aaron Tan Tuck Choy
School of Computing
National University of Singapore

Lecture 11: Sequential Logic
Latches & Flip-flops
 Introduction
 Memory Elements
 Pulse-Triggered Latch
 S-R Latch
 Gated S-R Latch
 Gated D Latch

 Edge-Triggered Flip-flops





S-R Flip-flop
D Flip-flop
J-K Flip-flop
T Flip-flop

 Asynchronous Inputs
CS1104-11

Lecture 11: Sequential Logic:
Latches & Flip-flops

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Introduction
 A sequential circuit consists of a feedback path, and employs some memory elements.
Combinational
outputs

Memory outputs

Combinational logic Memory elements External inputs

Sequential circuit = Combinational logic + Memory Elements
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Introduction

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Introduction
 There are two types of sequential circuits:
 synchronous: outputs change only at specific time
 asynchronous: outputs change at any time

 Multivibrator: a class of sequential circuits. They can be:
 bistable (2 stable states)
 monostable or one-shot (1 stable state)
 astable (no stable state)

 Bistable logic devices: latches and flip-flops.
 Latches and flip-flops differ in the method used for changing their state.
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Introduction

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Memory Elements
 Memory element: a device which can remember value indefinitely, or change value on command from its inputs.
Memory
element

command

Q

stored value

 Characteristic table:

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Command
(at time t)

Q(t)

Q(t+1)

Set

X

1

Reset

X

0

Memorise /
No Change

0
1

0
1

Memory Elements

Q(t): current state
Q(t+1) or Q+: next state

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Memory Elements
 Memory element with clock. Flip-flops are memory elements that change state on clock signals.
Memory
element

command

Q

stored value

clock

 Clock is usually a square wave.
Positive pulses

Positive edges
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Negative edges

Memory Elements

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Memory Elements
 Two types of triggering/activation:
 pulse-triggered
 edge-triggered

 Pulse-triggered
 latches
 ON = 1, OFF =