Switch
Design
Module
2
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pp. 4-15, pp. 295-304 & the file at the weblink on original
Reading
Assignment page |
Work Problems 1, 3, 5 |
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Basic Switches
Switch Logic
- Series
network, truth table, Boolean expression, AND logic
- Parallel
network, truth table, Boolean expression, OR logic
- more complex examples, truth tables, and introduction of Boolean expressions
Transistor Implementation of Switches
- Voltage levels, threshold voltages, high/low impedance, 0,1, True, False
- n-type, truth table, logic level 0/1 transmission
- p-type, truth table, logic level 0/1 transmission
- transmission gate, operation, truth table
- example
Examples of Standard Functions
- Implementation, Boolean expressions, and truth tables for INVERTER, NAND, NOR, AND, OR
- More complex examples
- Identify the Pull-up network + pull-down network to implement Boolean function: Demorgans theorem
- Number of transistors/gate as a function of the number of inputs
DeMorgan's Equivalence
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A |
B |
Out |
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0 |
0 |
1 |
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0 |
1 |
1 |
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1 |
0 |
1 |
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1 |
1 |
0 |
- The truth table may be perceived as an AND function: output=0 iff A=1 AND B=1
- The truth table may be percieved as an OR function: output=1 iff A=0 OR B=0
Switch Networks: Implementation of Boolean Functions
- Pull-up network: connect output to Vdd or float, use p-type transistors
- Pull-down network: connect output to Gnd or float, use n-type transistors
- Possible values: short (bad!), floating, pull-up or pull down, extended signal value set (0,1,Z,U)
- Structure of an inverter
- Series-parallel structure of pull-up and pull-down networks and introduction to duality
Implementation of Boolean functions in switch networks
- Structure of the implementation
- Strategy:
design the networks for
(pull-up) and 
(pull-down) - Notion of dual network
- Relationships: Pull-up network à Boolean expression à Complement expression (DeMorgan’s Theorem) à Pull-down network
- Incomplete circuits, shorts, floats, and availability of complements
Analysis of Switch Networks
- Transformations from switching networks to truth tables and Boolean expressions
- Examples
Synthesis of Switch Networks
- Transformations from truth tables and Boolean expressions to switch networks
- Examples
Physical Properties of Switches
- Propagation delays, rise time and fall time – there really are no digital devices!
- Gate loading: standard load, input and output loads
- Basic elements of energy consumption
Examples
The following applets were developed by the TECH group in
the Department of Computer Science at the
n-type and p-type switches
This example animates the operation of an n-type and p-type switch. Toggle the inputs of each switch and note the effect in the switch. Pay attention to the legend at the bottom of the figure to interpret the figure.
Inverter
The inverter operation can be understood by toggling the value of the input signal A and examining the value (color coded) of the output signal Y. Note how the value of the Y is pulled up to VCC or pulled down to GND depending on the value of A and the switches that are turned on.
Two Input NOR Gate
Experiment with the two-input NOR gate. Toggle the two input signals. See how they affect the switches in the pull-up network and the pull-down network. The result will be to pull-up Y to VCC or down to GND in accordance with the adjacent truth table. Note the series structure of the pull-up network and the parallel structure of the pull-down network.
Two and Three Input NAND Gates
Experiment with the two-input NAND gate. Toggle the two input signals. See how they affect the switches in the pull-up network and the pull-down network. The result will be to pull-up Y to VCC or down to GND in accordance with the adjacent truth table. Note the series structure of the pull-up network and the parallel structure of the pull-down network. You can also experiment with a three-input NAND gate.
Complex Switch Network
This is an example of a more complex network (AND-OR-INVERT). Develop an intuition of how these switch networks function by toggling the input signals and determine how the switches interact to set the value of the output signal.