Decoders, Encoders, and Muxes, Oh My!

Tom Kelliher, CS 220

Oct. 1, 2003

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Assignment

Read 3.8--9.

From Last Time

Combinational design example.

Outline

1. Decoders

2. Encoders

3. Muxes

Coming Up

Binary addition and subtraction.

Decoders

Block diagram (diagram a 3-8 decoder):

A circuit with n inputs. The inputs are interpreted as a binary number and used to select one of output lines.

1. Most common use: Address decoders for RAMs and register files.

2. Decoder expansion example: design a 5-to-32 using 5 3-to-8 decoders.

3. Example use: Design a BCD to seven segment decoder using a 4 to 16 decoder and OR gates.

Encoders

Inverse of a decoder: inputs; n outputs.

1. What happens if multiple inputs are high?

2. Most common uses: priority encoders for interrupt controllers; ``hit'' logic for caches.

3. Design example: Four input priority encoder with a ``Valid'' output.

Muxes

Used to select one of inputs. One way switch.

Block diagram (draw a 4-1 mux):

1. Most common uses: RAM, register file data selection circuits; Boolean function generators.

2. What's a quad 2-1 mux?

3. Design example: Implement a three input Boolean function using an 8-1 mux. No additional logic.

   Four input Boolean function using an 8-1 mux and an inverter?

   Five input Boolean function?