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Digital Logic Topics. Brief notes to preface the laboratory exercise. Brief summary of characteristics. Digital logic Electronic elements for processing digital (numeric) information Normally bi-state, 1 or 0 digital values represented by two discrete electrical signals

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digital logic topics

Digital Logic Topics

Brief notes to preface the laboratory exercise

BAE 5413

brief summary of characteristics
Brief summary of characteristics
  • Digital logic
    • Electronic elements for processing digital (numeric) information
    • Normally bi-state, 1 or 0 digital values represented by two discrete electrical signals
    • Our concern is primarily interfacing with logic inputs and outputs
  • Important terms/issues
    • Fan-out - the amount of output drive capacity compared to input drive requirement
    • Open collector - an output that is configured as a switch to reference voltage rather than a voltage level signal
    • Propagation delay - the amount of time between a change in input state and a change in output state.

BAE 5413

visualization as a comparator
Visualization as a comparator
  • Comparator
  • Vout = +5 if Vin 1 > Vin 2 else Vout = 0
  • Devices are available designed for the comparison purpose. They are in general faster and better suited for this purpose than a conventional op-amp.
  • Devices actually used in digital logic are much simpler internally than an Op Amp

BAE 5413

truth tables
Truth Tables
  • Consider the following and gate
  • A truth table “enumerates” the output state for every possible input state. See Example:

Input Output

A B C

0 0 0

1 0 0

0 1 0

1 1 1

BAE 5413

voltage levels
Voltage levels
  • Common logic types (families)
    • TTL
      • Transistor-Transistor Logic - Classically has 0 and 5 V nominal logic levels. lower voltage logic is evolving
      • High power consumption, simple, low cost
    • CMOS
      • Low power consumption. (current flows primarily during transition and is low when at a particular state)
      • Static sensitive
      • More complex than TTL
      • Fabricated in wider voltage ranges (4000 series)

BAE 5413

voltage levels1
Noise Margins

Comparison among logic families

Voltage levels

Input

Output

BAE 5413

slide7
TTL
  • Gate Electronic structure

TTL Bipolar Gate

TTL Open Collector Gate

BAE 5413

cmos logic
CMOS - Logic

Low power

3 - 18 V logic levels

Static Sensitive

Gate Electronic Structure

Example gate circuit

CMOS Logic

BAE 5413