Decoders and Encoders

Objectives

1. Discuss how to implement a decoder
2. Discuss and implement an encoder

Notes

• Build a truth table for decoder outputs (2 input bits please)
• This is simply just each of the minterms.
• An encoder takes 2ⁿ inputs and produces n outputs
• But only one of the input lines can be active at a time.
  • The input line represents a number 0 - 2ⁿ-1
  • The output lines represent the bits that number is coposed of.
• In addition, the encoder may have a valid bit output.
  • This is used to record that only one of the input lines is high.
• A priority encoder can have more than one input bit high.
  • It returns the value of the "highest" line
  • The 74147 is a chip that implements this.
    • Note, the logic is backwards, ie the not of the line set is output.
    • 
    • The digital file
  • This can be implementd with a buch of don't cares
  •