Microsoft Word - Digital Logic Design v_4_6a

3.6. Glitches and Their Causes

 A glitch is a momentary error condition on the output caused by unequal signal paths delays in the
circuit. This may appear as an additional pulse high or low that will go away once the circuit reaches
a steady state condition (after the signal has propagated through the circuit completely)

 Glitches can occur when a hazard condition exists (Function and Logic Hazards)

1) Functional Hazard

 Exists when there is a problem due to two or more inputs are changing at the same time.

 May be static when output is not changing or dynamic when the output changes.

 Cannot be removed by additional circuitry

2) Logic Hazards

(1) Exists when there is a problem due to a single input change.

(2) May be static when output is not changing or dynamic when the output changes.

(3) Can be removed by additional circuitry

 Example: Use Function F(A,B,C)= A.C+B.C to show both function and logic static hazards

”Static means before and after the Glitch the output is the same”

(User DeMorgan’s Theorem to use only NAND Gates)

Step 1. Generate a K-map and draw a schematic. Both of these drawings will assist in

identifying unequal propagation path and, therefore, delays through the circuit.

Step 2. Take a look at any logic static hazards that exist and if they may cause a glitch.

Note: when a single input is changing.

00

01

11

10

AB C 0 1 A^

C

B

F

00 01

02

13

16 17

14

05

C1