Flip Flops 3.1

Latches and Flip-Flops 3

©Paul GodinCreated September 2007

Last Edit Aug 2013

Flip Flops 3.2

Edge-Triggered Devices

◊ Up to now we have looked at two digital values or states: 1 and 0

◊ There are also two dynamic digital values or states: ◊ rising edge (positive edge)◊ falling edge (negative edge)

posi

tive e

dge

negativ

e e

dge

1

0 0

Flip Flops 3.3

Edge-Triggered Devices

◊ Inputs to some devices will respond only to a dynamic state.

◊ This is known as an edge input or a clock input.◊ The symbol is a triangle at the input.

Positive Edge Negative Edge

Flip Flops 3.4

Edge-Triggering Notation

◊ Positive Edge:

◊ PGT (positive going transition)

◊ PT (positive transition)◊ P

◊ Negative Edge:

◊ NGT (negative going transition)

◊ NT (negative transition)◊ N

Flip Flops 3.5

Advantages of Edge-Triggered Devices

◊ Edge-Triggered devices are important for many applications, including:◊ Counters

◊ Clocks◊ Timers◊ Digital meters◊ Sequential circuits◊ Mathematical operations, …

◊ Shift Registers◊ Memory◊ Data communications◊ Logic operations◊ Mathematical operations, …

Flip Flops 3.6

Edge-Triggered D Flip Flop

◊ The edge-triggered D-Flip Flop will only read its input when the edge is received.

◊ “With an enabling edge, the Q output follows the D input”.

D Q

QClock

En D Q Q’ Mode

0 0 1 Reset

1 1 0 Set

1 X Q Q’ Hold

0 X Q Q’ Hold

X Q Q’ Hold

This is one memory cell in computer applications

Flip Flops 3.7

Edge-Triggered SR

◊ The edge triggered SR is not very popular.

◊ The only difference between the SR Latch and the edge-triggered SR is that it will read its input only when it receives the active edge.

Flip Flops 3.8

Edge-Triggered SR Flip Flop

Clk S R Q Q’ Mode

0 0 Q Q’ Hold

0 1 0 1 Reset

1 0 1 0 Set

1 1 0 0 Invalid

0 X X Q Q’ Hold

1 X X Q Q’ Hold

X X Q Q’ Hold

S

R

Q

Q

Clk

Flip Flops 3.9

Edge-Triggered JK Flip Flop

◊ The JK Flip Flop is a device that functions like an edge-triggered SR latch.

◊ The only exception: toggle state on two active inputs.

◊ Toggle means go to the complimentary (opposite) output state. This is a very useful function.

Flip Flops 3.10

Edge-Triggered JK Flip Flop

Clk J K Q Q’ Mode

0 0 Q Q’ Hold

0 1 0 1 Reset

1 0 1 0 Set

1 1 Q’ Q Toggle

0 X X Q Q’ Hold

1 X X Q Q’ Hold

X X Q Q’ Hold

J

K

Q

Q

Clk

Flip Flops 3.11

Toggling

Toggling has important applications including counters and frequency dividers.

A “T” Flip-Flop has a clock input only. The output will toggle on an input edge.

TQ

QClk

Flip Flops 3.12

Exercise 1: Edge-Triggered D Flip Flop

Complete the table

D Q

QClock

En D Q Q’ Mode

0

1

1 X

0 X

X

Flip Flops 3.13

Exercise 2: Edge-Triggered JK Flip Flop

Clk J K Q Q’ Mode

0 0

0 1

1 0

1 1

0 X X

1 X X

X X

J

K

Q

Q

Clk

Complete the table

Flip Flops 3.14

Exercise 3: D Flip Flop timing diagramComplete the timing diagram

D

Q

Q

Clk

D Q

QClock

Flip Flops 3.15

Exercise 4: JK Flip Flop timing diagram

J

K

Q

QClk

Complete the timing diagram

J

K

Q

Q

Clk

Flip Flops 3.16

Exercise 5: JK Flip Flop timing diagramComplete the timing diagram

J

K

Q

Q

Clk

J

K

Q

QClk

Flip Flops 3.17

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