Digital Logic Topics

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# Digital Logic Topics - PowerPoint PPT Presentation

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

Brief notes to preface the laboratory exercise

BAE 5413

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
• 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
• 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
• 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

Noise Margins

Comparison among logic families

Voltage levels

Input

Output

BAE 5413

TTL
• Gate Electronic structure

TTL Bipolar Gate

TTL Open Collector Gate

BAE 5413

CMOS - Logic

Low power

3 - 18 V logic levels

Static Sensitive

Gate Electronic Structure

Example gate circuit

CMOS Logic

BAE 5413