3-2-4. Combinational Logic: Decoders

Decoders
Example: VHC138
A decoder converts binary information from the N coded inputs to a maximum of 2^N unique outputs. It is commonly used to increase the number of ports and generate chip select signals.

Operation of a decoder
The following shows a logic symbol, truth table, and timing diagram of a 3-to-8 decoder, i.e., a decoder with three inputs and eight outputs.

The following shows how a 3-to-8 decoder is used to generate eight chip select signals from three inputs (A, B, and C). When A, B, and C are all Low, only the /Y0 output provides a logic Low, causing IC0 to be selected.
This figure indicates that, with a combination of three inputs, an arbitrary chip can be selected from up to eight chips.