EE 201: Latchtes and fl ip- fl ops Steven Bell 12 February 2019 By - - PowerPoint PPT Presentation

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EE 201: Latchtes and fl ip- fl ops Steven Bell 12 February 2019 By the end of class today, you should be able to: Compare and contrast an SR latch , D latch , and D fl ip- fl op Given a circuit containing a latch or fl ip- fl op, draw a timing

SLIDE 1

EE 201: Latchtes and flip-flops

Steven Bell 12 February 2019

SLIDE 2

By the end of class today, you should be able to:

Given a circuit containing a latch or flip-flop, draw a timing diagram to illustrate how and when the output changes. Compare and contrast an SR latch, D latch, and D flip-flop Sketch the general strucutre of a sequential circuit Explain why flip-flops are often preferred over latches for sequential circuit design.

SLIDE 3

A circuit that counts

SLIDE 4

S R Q Q

NOR NOR

This circuit is stable

SLIDE 5

S R Q Q

NOR NOR

S R Q Q

NOR NOR

S R Q Q

NOR NOR

When S goes high, the output (Q-bar) goes low. This causes the other output (Q) to go high.

SLIDE 6

S R Q Q

NOR NOR

S R Q Q

NOR NOR

S R Q Q

NOR NOR

S R Q Q

NOR NOR

Now S goes back low, but Q remains high. Magic!

SLIDE 7

S R Q Q

NOR NOR

Now, what happens when R goes high?

SLIDE 8

S R Q Q

NOR NOR

S R Q Q

NOR NOR

S R Q Q

NOR NOR

When R goes high, Q goes low. And then Q-bar goes high, making things stable again.

SLIDE 9

S R Q Q

NOR NOR

S R Q Q

NOR NOR

S R Q Q

NOR NOR

S R Q Q

NOR NOR

Now when R goes low, the state is remembered.

SLIDE 10

S R Q Q

NOR NOR

What happens when we start?

SLIDE 11

S R Q Q

NOR NOR

S R Q Q

NOR NOR

S R Q Q

NOR NOR

SLIDE 12

SLIDE 13

D latch

SLIDE 14

D flip-flop

SLIDE 15

The sequential abstraction

We use only flip-flops All flip-flops are driven by a single clock

SLIDE 16

Comparison

SLIDE 17

Practice time

http://172.104.217.120:8000/

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1. Read the book (4.4-4.5, 5.4) and complete the pre-class quiz

For Thursday

2. Complete the online homework problems and submit via provide