MSI components

Summary

Active high/low

• active high - normal outputs: true is high
• active low - negated outputs: true is low

Constructing decoders

• use two smaller decoders with the additional input and its negation to the enables

watch for whether the decoders are 1-enable or 0-enable

Constructing multiplexers

• use the MSB to discriminate between the outputs of two smaller multiplexers on the other bits

Concept

Decoders

• n-bit BCD to 2ⁿ ouputs
• each output is a minterm

• 2:4 decoder:

• 2:4 decoder with 1-enable:

Encoder

• 2ⁿ inputs to n-bit BCD
• only one input can be 1
• 4:2 encoder:

• 4:2 encoder with 1-enable:

Priority encoders

• encoders that allows for multiple 1s in the input
• encode the MSB to the output

Demultiplexer

• 1-bit in, n-bit selector to 2ⁿ ouputs
• equivalent to a decoder with enable, using the enable as the input

Multiplexer

• 2ⁿ inputs to n-bit BCD, n-bit selector

• 4:1 multiplexer:

• 4:1 multiplexer with 0-enable:

Application

Implementing functions with decoders

• join the select outputs by some logic gate

using the convention that high is 1

Implementing functions with multiplexers

• supply 1 if the input corresponds to a minterm in the function

74151A 8:1 multiplexer

• 0-enable

Implementing functions using smaller multiplexers

• 8:1 multiplexer using a 4:1