ECED2200 Digital Circuits Time Response & Hazards 18/07/2012 - - PowerPoint PPT Presentation

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ECED2200 Digital Circuits Time Response & Hazards 18/07/2012 Colin OFlynn - CC BY-SA 1 General Notes See updates to these slides: www.newae.com/teaching These slides licensed under Creative Commons

SLIDE 1

ECED2200 – Digital Circuits

Time Response & Hazards

18/07/2012 Colin O’Flynn - CC BY-SA 1

SLIDE 2

General Notes

See updates to these slides: www.newae.com/teaching
These slides licensed under ‘Creative Commons Attribution-ShareAlike 3.0

Unported License’

These slides are not the complete course – they are extended in-class
You will find the following references useful, see

www.newae.com/teaching for more information/links:

– The book “Bebop to the Boolean Boogie” which is available to Dalhousie Students – Course notes (covers almost everything we will discuss in class) – Various websites such as e.g.: www.play-hookey.com – The book “Contemporary Logic Design”, which was used in previous iterations of the class and you may have already

18/07/2012 Colin O’Flynn - CC BY-SA 2

SLIDE 3

Time Response of Gates

18/07/2012 Colin O’Flynn - CC BY-SA 3

SLIDE 4

NOT Gate

A Y 1 1

Y=A

18/07/2012 Colin O’Flynn - CC BY-SA 4

SLIDE 5

NOT Gate

A Y 1 1

Y=A

18/07/2012 Colin O’Flynn - CC BY-SA 5

SLIDE 6

Time Response

18/07/2012 Colin O’Flynn - CC BY-SA 6 Source: http://www.nxp.com/documents/data_sheet/74HC_HCT00.pdf

SLIDE 7

Typical Values

18/07/2012 Colin O’Flynn - CC BY-SA 7

74LS00: 74HC00: 74F00:

SLIDE 8

USEFUL GATE DELAYS

18/07/2012 Colin O’Flynn - CC BY-SA 8

SLIDE 9

Pulse Shaper

18/07/2012 Colin O’Flynn - CC BY-SA 9

A B C D Y

SLIDE 10

Ring Oscillator

18/07/2012 Colin O’Flynn - CC BY-SA 10

A B C D Y

SLIDE 11

NOT-SO-USEFUL GATE DELAYS

18/07/2012 Colin O’Flynn - CC BY-SA 11

SLIDE 12

Glitches

18/07/2012 Colin O’Flynn - CC BY-SA 12

SLIDE 13

Hazards

Circuits with potential for a glitch have a hazard

18/07/2012 Colin O’Flynn - CC BY-SA 13

SLIDE 14

Types of Hazards

18/07/2012 Colin O’Flynn - CC BY-SA 14

Static 1-Hazard Static 0-Hazard Dynamic Hazard

SLIDE 15

What Creates Hazards?

Single-bit (variable) input changes only!

18/07/2012 Colin O’Flynn - CC BY-SA 15

SLIDE 16

Analyzing Hazards & Fixing Them

18/07/2012 Colin O’Flynn - CC BY-SA 16

A•B•C A•B•C A•B•C A•B•C A•B•C A•B•C A•B•C A•B•C A B+B C f   

SLIDE 17

Hazard Free Form

18/07/2012 Colin O’Flynn - CC BY-SA 17

SLIDE 18

Other Hazards?

In two-level networks (assuming complements are available) synthesized in sum-of-product form, removal of state 1-hazard means static 0- hazards and dynamic hazards also removed.

18/07/2012 Colin O’Flynn - CC BY-SA 18

SLIDE 19

Multilevel Hazards

1. Simplify using some basic Boolean laws to get

to two-level form

– CANNOT use complement laws or simplification laws derived from it

2. Using K-Map derive hazard-free form
3. Do not convert back into multilevel form –

difficult to remove dynamic hazards

18/07/2012 Colin O’Flynn - CC BY-SA 19

SLIDE 20

Example

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f=A•B•C+(A+D)•(A+C) f=A•B•C+A•A+A•C+D•A+D•C

SLIDE 21

Example

18/07/2012 Colin O’Flynn - CC BY-SA 21

f=A•B•C+A•A+A•C+D•A+D•C

A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D A•B•C•D

SLIDE 22

References

See class notes “Hazards”

18/07/2012 Colin O’Flynn - CC BY-SA 22