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Lecture 13 - PowerPoint PPT Presentation

Lecture 13. ES 210 Latches 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 output changes as soon as the input changes – this is not desirable if you do not

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Lecture 13

ES 210

Latches and Flip-Flops

Jack Ou, Ph.D.

Caution for a D latch: once a clock enables a D latch, the output

changes as soon as the input changes – this is not desirable if you do not

want the output to change continuously and all the latches use a common clock.

Q=Y

hold

0

1

Y=D

hold

1

0

2

1

The value that is produced

at the output of the flip-flop

is the value that was stored

in master stage immediately

before the negative edge

Occurred.

OUT=X

Not enough

time for D→Y→Q

Q will hold steady

Y=D

1: Track

1:hold

1:hold

CK of latch 1

CK of latch 2

2: Hold

2:track

2:track

Q=Y

X

2

1

OUT=X

2: Track

2:hold

2:hold

CK of latch 2

CK of latch 1

1: Hold

1:track

1:track

X=IN

• 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.

• 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.

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

1

0

1

0

CLK =0, maintain the present state

D=0 as Clk=0→ 1

1

0

1

0→1

1 → 0

0

1

Q changes 0

D=1 as Clk=0→ 1

0

1

1 → 0

0→1

1→ 1

1

0

Q changes 1

D=0→ 1 as Clk=1

1

S’

S

1

S’

The flip-flop is unresponsive to changes in D

0 → 1

1

Please explore different possible value of S on your own.

This will work even for S=R=1 and S=R=0.

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.

1

0

1

1

0

Typo in the book. Should be 1 instead.

D=JQ’+K’Q

Positive edge D flip-flop

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.

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

D=TQ’+T’Q

If T=1, D=Q’

If T=0, D=Q.

Q is updated with D at the next rising edge.

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