[논리설계] 13. Schematics Rules, contention x, floating z

1. From Logic to Gates

 

2. Circuit Schematics Rules

•Inputs on the left (or top)

•Outputs on right (or bottom)

 

•Gates flow from left to right

•Straight wires are best

•Wires always connect at a T junction

•A dot where wires cross indicates a connection between the wires

 

•Wires crossing without a dot make no connection

 

 

📒Draw a schematic of the follwing equation

 

3. Multiple-Output Circuits

•Example: Priority Circuit

–Output asserted corresponding to most significant TRUE input

-Priority Circuit Hardware

-Don't Cares

 

4. Contention: X

1) Contention: circuit tries to drive output to 1 and 0

–Actual value somewhere in between

–Could be 0, 1, or in forbidden zone

–Might change with voltage, temperature, time, noise

–Often causes excessive power dissipation

 

 

2) Warnings:

–Contention usually indicates a bug.

–X is used for “don’t care” and contention - look at the context to tell them apart

 

5. Floating Z

1) Floating, high impedance, open, high Z

2) Floating output might be 0, 1, or somewhere in between

–A voltmeter won’t indicate whether a node is floating

 

Tristate Buffer

3) Tristate Busses

•Floating nodes are used in tristate busses

–Many different drivers

–Exactly one is active at once