Decoders

Objectives

1. Discuss decoders

Notes

• This is Carpinelli, chatper 4
• Decoder can have several meanings in this area
  • A decoder is a n input, 2ⁿ output device
  • A BCD to decimal decoder/driver is a circut designed to take some input and drive another device, like the 74x48 BCD to 7-segment decoder/driver
• We learned (somewhat) how to build the second.
• We need to learn about the first.
• 
• Sometimes we have an enable (or enable not) line as well.
• What would we use this for?
• Rmember in the register discussion, we might have a bank of registers.
  • Let's build a Register File
  • Each register is 8 bits.
  • We have four registers.
  • We have a common clock.
  • We will use a decoder to select the register
    • The input to the decoder will be the "address" of the register we want to write to.
    • This will probably come from the instruction.
  • 
  • Note, this is only 1/2 of the register file
    • We also need to control what register is being output, but we need a mulitplexor (mux) for that.
• Carpinelli has an interesting discussion on how we can build a n bit decoder out of multiple smaller decoders.
  • Here is the idea:
    • Use a decoder "up front" to select a bank of decoders
    • 
  • A tri-state device, sometimes called a driver
    • One input
    • One output
    • One control line
    • If the control line is high, the input data is passed to the output data
    • If the control line is low, no data is passed to the output.
    • 
  • In digital, I can buld my own components
    • file -> new embedded circuit
    • Create the circuit with input and output elements
    • Make sure you label all of the input/output elements
    • Save this
    • It will now be available in the "Custom" section
    • More on this later.
    • 
  • So my version of Carpinelli's circuit:
• The 74x238 is an 3 input, 8 output decoder
  • It has two enalble not (GA, GB lines and an enable (G) line.
  • It has data input lines A,B,C
  • It has eight (Y0 - Y7) output lines.
  • 
  • Data Sheet.
  • Note the 74x138 provides inverted output.
• Too late, I probably made a mistake using a tri-state device
  • Replace that with an and gate in class.
  • Tri-states will have NO OUTPUT,
  • but we want either a 0 or a 1
  • And will accomplish this.
• Decoder examples, but you need this file too.