Timing Diagrams

• Shows signal state (1 or 0) as a function of time• Dependent variable (horizontal axis) used for

time• Independent variable (vertical axis) used for

signal state

NAND gate Example Timing Diagram

Device Models

• Model: An arbitrarily defined set of characteristics used to describe the operation of a device.

inverter

Idealized model Model with gate delays

tpLHtpHL

Propagation Delays

• Present in all digital devices• Prop delays vary based on

– device type– device family – device operating conditions

• Rising signal delays (tpLH) and falling signal delay times (tpHL) are usually different for a given device

Timing Diagram Annotation

• Draw reader’s attention to important conditions in circuit

• Help them find the “trees” instead of getting overwhelmed with the “forest”

Timing Diagram Annotation

arrows show simple cause and effect relationship a more complex circuit

• Draw reader’s attention to important conditions in circuit

• All timing diagrams should be annotated!

•Did we mention that………All timing diagrams should be annotated!?

Circuit Simulation

• Allows for testing a design before actual circuit implementation

• Simulation represents a “best guess” of how the circuit will operate

• Accuracy of simulation compared to actual circuit based on quality of models used

• B2 Spice A/D Lite used for Experiment 4

Available for FREE: http://www.beigebag.com/adv4_lite.htm

Glitches

• Unwanted momentary error conditions in circuit

Glitches

• Glitches have many causes– cross talk – EMI (Electro-Magnetic Interference)

– switching noise– Logic hazards in circuit design

• Experiment 4 glitches caused by device propagation delays in circuit elements

Glitches: Experiment 4

• Caused by static logic hazards– change in single input variable causes

unexpected change in outputOnly InputA changes

1 0

1

1F(A,B,C) = F(1,1,1) = 1

“Static” – F stays @ 1

A BC

00 01 11 10

0 0 1 1 0

1 0 0 1 1

F(A,B,C) = F(0,1,1) = 1

Glitches: Experiment 4

“Logic Hazard Cover Term”

ABC

00 01 11 10

0 0 1 1 0

1 0 0 1 1

• Glitches prevented by including additional cover terms in K-map

Example shown in Experiment 4:

Experiment 4 Procedure Overview

• B2 Spice A/D Lite tutorial (DONE!!?)• Ring Oscillator analysis

• Static-1 logic hazard detection & removal

• Static-0 logic hazard detection & removal

Ring Oscillator

Include output ports for every gate output

…so you can observe how the circuit operates

How Do You Choose What Values (1/0) to Apply to Your Inputs To

Provoke a Possible Glitch??

• How many inputs should change simultaneously to induce a glitch?

• How can we determine which input signal levels may cause a glitch?

• Which direction should you change your one input signal? 1 0 or 0 1?

For Next Week:• Note which Logic Analyzer Model is at your

station (HP 1651A or HP 1663)

• Review the tutorial:

“An Intuitive Description of the Logic Analyzer”

• Read Experiment #5 to prepare for using the Logic Analyzer and the Xilinx Tools

– Be sure to download and review the correct Experiment #5 version for YOUR particular model of Logic Analyzer

• There are 2 different versions – one for each LA model.