Lecture 13

ES 210Latches and Flip-Flops

Jack Ou, Ph.D.

Using a Latch as a Memory Element

Caution for a D latch: once a clock enables a D latch, the outputchanges as soon as the input changes – this is not desirable if you do notwant the output to change continuously and all the latches use a common clock.

Uses of Flip-flops

D Flip-Flop

Negative Edge triggered D Flip-FlopClk=0

0 1

Q=Yhold

Negative Edge triggered D Flip-FlopClk=1

1 0

Y=D

hold

Negative Edge Triggered D Flip-Flop

CK of latch 1

CK of latch 2

OUT=X

Y=D

2: Hold

1: Track

1:hold

2:track

1 2

1:hold

2:track

Q=Y

Not enoughtime for D→Y →Q

Q will hold steady

The value that is producedat the output of the flip-flopis the value that was storedin master stage immediately before the negative edge Occurred.

Positive Edge D-Flop

1 2

CK of latch 2

CK of latch 1

X

X=IN

OUT=X

1: Hold

2: Track

2:hold

1:track

2:hold

1:track

Timing Diagram

Definition

• Setup time: the time that the incoming data must be stable before the clock arrives

• Hold time: the length of time that the data remains stable after the clock arrives for proper operation

• If the data is stable before the setup time and continues to be stable after the hold time, the flop will work properly.

• If the data arrives within the period designated by the setup and hold times, the flop may or may not capture the correct value.

CLK-Q

• The delay from the time that the clock arrives to the point that the output stabilizes.

• In reality the data must arrive at the setup time before the clock hits and the output is valid after the CLK-Q delay.

Question

• How would you change a negative edge triggered flip-flop to a positively edge triggered flip-flop?

D-Type Positive Edge Triggered Flip-Flop (CLK=0)

0

0

1

1

CLK =0, maintain the present state

D-Type Positive Edge Triggered Flip-Flop

0

0→ 1

1

1 → 0

Q changes 01

10

D=0 as Clk=0→ 1

D-Type Positive Edge Triggered Flip-Flop

1

0→ 1

1 → 0

1 → 1

Q changes 10

01

D=1 as Clk=0→ 1

D-Type Positive Edge Triggered Flip-Flop

0 → 1

1

S

The flip-flop is unresponsive to changes in D1

1

D=0→ 1 as Clk=1

S’

S’

Please explore different possible value of S on your own.This will work even for S=R=1 and S=R=0.

Symbol of D Flip-Flops

reset and preset

• When power is first turned on, the state of the flip-flops is unknwon.– Reset is used to initialize the output to a

0.– Preset is used to initialize the output to

a 1.

Reset Feature

0

1

1

1

0

D Flip-flop with reset

Typo in the book. Should be 1 instead.

JK Flip-FlopsD=JQ’+K’Q

The next value of D is determined by JQ’+KQ.At the rising edge of D Flip-flop, Q is updated with the value of D.

Positive edge D flip-flop

D=JQ’+K’Q

• J=1,K=1→D=Q’• J=0, K=0 →D=Q• J=0, K=1 →D=0• J=1, K=0 →D=Q’+Q=1

Differences Between a D Flip-Flop and JK Flip-Flop

T Flip-Flop

T Flip-Flop from D Flip-Flop

D=TQ’+T’Q

If T=1, D=Q’If T=0, D=Q.

Q is updated with D at the next rising edge.

DFF with reset

Phase Frequency Detector

Delay B by 10 ns

Delay A by 10 ns

Clk_A=20 MHz, Clk_B=21 MHz

Clk_A=21 MHz, Clk_B=20 MHz

Clk_A=20 MHz and Clk_B=20 MHz