Some Useful Circuits

• A decoder is a circuit that given n input lines, selects between 2ⁿ output lines.
  • Think of it as
    • Each of 2ⁿ lights is assigned a number.
    • If I input the number, light n is lit up.
  • Why n and 2ⁿ?
  • The normal image is:
    • 
  • But in tkgate they have a pwer switch.
    • 
  • We use a decoder to select among different items.
    • Very frequently memory or registers.
  • So how would we implement a 3 bit decoder?
• An encoder is the opposite.
  • We will use these less frequently, but it is good to know.
  • There are 2ⁿ input bits.
  • And n output bits.
• A multiplexor is a device with
  • n data inputs, usually a power of 2.
  • 1 control input but it is log₂ bits wide.
  • 1 data output.
  • 
  • 
• Let's build a component of the basic mips architecture.
  • MIPS has 32 general purpose registers available.
    • This is somewhat of a lie, look at the green sheet.
      • Register 0 is just a source for 0, and the bit bucket.
      • register 1 is reserved for the assembler.
      • Registers 26-31 are reserved for predefined uses
    • This is sometimes called the register file.
  • For simplicity, let's build a 4 register file.
  • This circuit will have one data input and one data output.
    • The input will be an 8 bit value to store. (Data)
    • The output will be an 8 bit value stored in a register. (Output)
  • It will have three control lines.
    • A 2 bit output register line, used to select the register for output. (Read Register)
    • A 2 bit store register line, used to determine where the data will be stored. (Write Register)
    • A clock, which stores the input data when the clock switches from low to high. (clock)
  • And two pseudo control lines
    • Enable not.
    • Clear not.
  • Before we begin, we need a little knowledge of registers.
    • Registers have
      • A word sized data in line.
      • A word sized data out line.
      • A clock line.
      • An enable not line
      • A clear not line.
      • 
    • When the clock goes from low to high, the data on the data in line is stored in the register.
      • But the data must be stable on the input line for setup time.
      • And nothing must change for hold time.
      • The data is available in clock_q time.
  • So we will
    • Need a decoder to select the register where we will store values
      • The Write_Register line will be the input to the decoder.
      • Four output lines will, we will and each with the clock input line and feed the individual register clocks.
      • This will cause the clock pulse to hit only one register
    • Need a mux to select the output
      • The four register outputs will be fed as input into the mux.
      • The Read_Register line will be used to select the desired register.
  • 
  • TKGate implementation