COMBINATIONAL LOGIC II Flashcards
Give an example of an application of the decoder
A decoder with an enable input is referred to as a
decoder/demultiplexer.
The truth table of demultiplexer is the same with decoder.
*See notes for image of the demultiplexer
Give an example of an application of the decoder
A decoder with an enable input is referred to as a
decoder/demultiplexer.
The truth table of demultiplexer is the same with decoder.
*See notes for image of the demultiplexer
Explain and illustrate a possible way to implement a 4-to-16 decoder
- When w = 0, the top decoder is enabled and the other is disabled.
- The bottom decoder outputs are all 0’s, and the top eight outputs generate minterms 0000 to 0111.
- When w = 1, the enable conditions are reversed:
- The bottom decoder outputs generate minterms 1000 to 1111, while the outputs of the top decoder are all 0’s.
*See page 59
Describe and illustrate the use of a decoder to create a traffic light
Using a 2-4 decoder, the circuit which generates traffic light combinations is as follows.
To complete the traffic light controller, we just need to
make the inputs i0 and i1 cycle through the binary representations of the numbers 0-3.
*See page 60
Describe what an encoder is and give an example
An encoder is a digital circuit that performs the inverse operation of a decoder.
An encoder has 2n (or fewer) input lines and n output lines.
An example of an encoder is the octal-to-binary encoder
*See page 61 for truth table
What are some of the ambiguities of the octal-to-binary encoder
If two inputs are active simultaneously, the output produces an undefined combination e.g D3 and D6 are 1 simultaneously. We can establish an input priority to ensure that only one input is encoded.
Another ambiguity in the octal-to-binary encoder is that an output with all 0’s is generated when all the inputs are 0; the output is the same as when D0 is equal to 1.
Describe and illustrate the priority encoder
A priority encoder is an encoder circuit that includes the priority function.
The operation of the priority encoder is such that if two or more inputs are equal to 1 at the same time, the input having the highest priority will take precedence.
- Addresses the ambiguities of the octal-to-binary encoder
*See page 64 for illustration
Describe the functioning of the Valid bit indicator
Set to 1 when one or more inputs are equal to 1. If all inputs are 0, there is no valid input and V is equal to 0.
V=0–>no valid inputs
V=1–>valid inputs
*See page 63 and 64
What is the benefit of using don’t care conditions in the truth table of the octal-to-binary truth table
X’s in output columns represent don’t-care conditions
X’s in the input columns are useful for representing a truth table in condensed form.
Instead of listing all 16 minterms of four variables.
*See page 63 and 64
How is priority assigned in the priority encoder
The higher the subscript number, the higher the
priority of the input
Input D3 has the highest priority, so, regardless of the
values of the other inputs
Describe and illustrate the output, truth table and K-Maps of the 7 bit display
A more useful application of combinational encoder design is a binary to 7-segment encoder.
*See page 65 and 66
Describe a multiplexer and illustrate its truth table, circuits
A multiplexer is a combinational circuit that selects binary information from one of many input lines and directs it to a single output line.
*See page 68
Illustrate:
2-input Multiplexer Design - NAND
4-to-1 Line Multiplexer
4-input Multiplexer Design - NAND
*See page 69-71
Describe and illustrate the rotary switch
The rotary switch, also called a wafer switch, is a mechanical device whose input is selected by rotating a
shaft.
The rotary switch is a manual switch that you can use to select individual data or signal lines simply by turning
its inputs “ON” or “OFF”.
*See page 72
Illustrate boolean function implementation using multiplexers
A more efficient method for implementing a Boolean
function of n variables with a multiplexer that has n-1
selection inputs.
*See page 74 and 75
