CSCI 255, ENGR 274, ECE 212 -- 19 September, 2000

Announcements

• Homework 2 solution
• Homework 3
• Test -- 28 September, 2000

Glitches and hazards

glitch

an unwanted pulse at the output of a combinational logic network

hazard

circuit with the potential for a glitch

Types of hazards

When a single-bit of input changes, the following should not happen:

Trying out an example

Z = A C + B C'

Multilevel networks

With more levels for propagation, the detection of hazards becomes harder. If hazard-free design is required, it may be best to stick to two-levels.

• F = A B C + (A + D)(A' + C)
  • Static 1-hazard at 1111 → 0111
  • Static 0-hazard at 1010 → 0010
• F = (A' B + B' C') (A B')
  • Dynamic hazard at 000 → 010

A theory of hazard-free transformations

• Start with the hazard-free two-level circuit.
• Transform with Boolean algebra, but without using complementarity
  • α α' = 0
  • α + α' = 1

Practical stuff

• the data sheet
• single-pole/single-throw switches
• LED's and current limiting resistors