VLSI Systems #5
Steering logic and Spice analysis.
P.J. Drongowski

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Steering logic
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  * Implement switching function as a chain of enhancement
    transistors in series.
  * This type of logic is sometimes called "steering" or
    "pass" logic.
  * Use multiplexer as an example.
  * Advantages:
    + Low power.
    + No need to distribute power and ground.
  * Precautions:
    + Beware of voltage drops across transistors.
    + Need restoring logic stage at end of chain.
  * Switching time is:
                  2
           Order(N  x R x C)
    where N is the number of transistors in the chain.

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Spice
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Methodology.

  * Write a "data book" description of the circuit listing inputs,
    outputs, truth tables, timing diagrams, assumptions on signals,
    (e.g., not high at the same time), etc.
  * Draw a schematic for the circuit.
  * Write a Spice description for the circuit.
    + Assign node numbers.
    + Define components.
    + Enable analysis, prints and plots.
  * Layout the circuit geometry
  * Measure capacitances more precisely and simulation again.

Describing a circuit in Spice.

  * File format.
  * Transistors.
  * Capacitors.
  * Resistors.
  * Voltage sources.

DC analysis.

  * Transfer curves of outputs versus inputs.
  * Remove capacitors, short inductors.
  * Need print or plot statements to view results.

Transient analysis.

  * Simulate behavior of system over time.
  * Very compute bound task (e.g., solving integrals, systems
    of linear equations, etc.)

Example.

Copyright (c) 1984 Paul J. Drongowski