Some Other Components

• We are going to look at some of the things that are standard components.
• I will get just a little out of order and present some sequential logic as well. (I want a register).
• In the end, we will build the shell of a Simple CPU.
• 
• This uses for three modules
  • A register
  • A Multiplexor/Demultiplexor
  • An Encoder/Decoder
• The register
  • This is way out of order so we will only use the unit here.
  • We will build one later.
  • This is a unit of memory.
  • Probably word size.
  • It has two data lines
    • The line marked D is an input data line.
    • The line marked Q is an output data line.
      • Q constantly (sort of) supplies the data stored in the register.
      • There are delays when the register is changing when the data might not be correct.
    • There are at two control lines in this register.
      • The en line makes the register active, ie respond to change.
      • The C line represents the clock.
    • By default, the register has some value.
    • To change the value
      • Put data on the D line (and have it stabilize)
      • Raise enable from low to high.
      • Raise the clock from low to high.
      • Hold enable and D for the required time.
      • Lower enable.
      • This is called an edge triggered device.
    • 
• De/Encoders
  • A decoder
    • n selector bits as input.
    • $2^n$ output bits.
    • The value of the input is reflected in the output.
    • 
    • This is relatively easy to build.
    • 
  • An encoder
    • Has n output bits
    • $2^n$ input bits.
      • But only one of these lines can be lit at a time.
    • The output represents the input line that was lit.
    • The circuit "encodes" the address of the line that is lit.
    • 
• Multiplexor (mux) and demultiplexor
  • A mux takes
    • $2^n$ input data lines
    • $1$ output data line.
    • 1 control line with $n$ bits.
    • Based on the value of the control line, the data from the selected input line is transferred to the output line.
    • 
    • And is not that hard to build
    • 
    • Figure B.3.6 on page 709 shows the implementation of a multi-channel MUX.
  • A demultiplexor does the opposite.
    • One data input line.
    • $2^n$ data output lines.
    • $n$ bit control line.
    • Sends the output to the data line selected by the control line.
    • 
• All of these circuits are available in parts.dig