Some Basic Circuits

• There are a set of circuits that we will be using as basic building blocks.
• Decoders/Encoders
  • n-bit input
  • 2ⁿ outputs.
  • This circuit essentially decodes the input and lights a corresponding output line.
  • Let us look at a 2 bit decoder
    • There is a two bit input line (I₁,I₀
    • There are 2² output lines, O₃ ···O₀

    • I1	I0	O3	O2	O1	O0
      0	0	0	0	0	1
      0	1	0	0	1	0
      1	0	0	1	0	0
      1	1	1	0	0	0

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  • Notice the slashed line to show more than one bit of data.
  • In tkgate we will need a few other items.
    • A dip switch allows us to have multiple switches in a single uint.
    • We can edit the proprieties to make it a 2 bit switch
      • Right click on it, select properties.
      • Go to Port
      • Highlight the port
      • Change the bit width to 2.
    • The tkgate decoder has an additional line, which seems to be an activation line.
    • 
    • To make our own version, we will need to use a wire merge
    • 
    • And we can build one out of and gates.
    • 
  • The opposite of a decoder is an encoder.
    • How would you implement one of these?
  • A decoder will be a key part of any memory system.
• A Multiplexer
  • Takes 2ⁿ input lines
  • Takes n control lines
  • Has a single output line.
  • It selects between the input lines, based on the control lines what to transfer to the output line.
  • 
  • We can implement one of these as well.
  • 
  • We often use a 2 to 1 mux with multiple data lines
  • 
  • We will use a multiplexer in our simple CPU.
    • For example, load the PC from either the bus or the PC + 1
• PLA - programmable logic arrays
  • Since everything can be represented as a sum of products
  • A special type of device called a PLA has been designed to handle this.
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  • A set of input lines, as well as the inversion of the this input comes into a AND plane
  • These lines can be anded together, and the result is passed to the OR plane
  • These lines are ored together and sent to the output.
  • Generally, the gates are not draw, just dots to indicate an and or or.
  • So our voting machine from before would become
  • 
  • We could reuse some of the minterms (if needed) for other results.
  • The gates are only laid down when the circuit is created