Registers

• Carpinelli discusses registers in chapter 7
  • I want registers to continue to make use of the circuits we are discussing
  • We will pick up more detail later.
• A register is a place to store data.
  • They are constructed from flip-flops, which are constrcuted from transistors.
• They represent a different form of circuit.
  • Combinational circuits are circuits where the ouptput is completely determined by the input.
  • Sequentioial circuits have a state, and the output is determined by the input and the state.
• The staet of a register is the data it holds.
• Registers have some timing issues, but we will discuss that in the future.
• Registers have
  • A data in line of n bits. (D)
  • A data out line of n bits (Q)
  • A clock input
  • They may also have one or more enable lines
    • data in
    • data out.
    • Which might be enable
  • They may have a reset line
• For the register in digital
  • Place data on the data in line.
  • Raise the enable line
  • Pulse the clock line.
  • When the clock goes from low to high, data on the data line will be stored in the register, if the chip is enabled.
  • 
• There are several register chips, the 74173 is one.
  • A 4 bit register.
  • 4 data in lines
  • 4 data out lines
  • A clock
  • A reset line
  • Ground and power.
  • Two output enable lines, need to be low.
  • Two input enable lines, need to be low
  • 
• So we could build a simple PC
  • I am using Digital's 4 bit adder.
  • 
  • But this is not the full operation of a pc.
    • We might need to load the pc from the bus (jump and branch instructions)
  • So I am going to add a mux
  • 
  • Note that there are now two control lines
    • PC source
      • 0: pc ← pc + 1
      • 1: pc ← bus_3..0
    • LoadPC
      • Load the pc from the data source slected above.
  • Note, Ben does not use this technique
    • He uses a counter.
      • 4 data input bits.
      • 4 data output bits.
      • A clock.
      • A load li
      • A direction line (count up or down)
      • A clear line
      • An enable line, only for
    • To count up
      • enable on, dir off, pulse clock
    • To cound down
      • Enable on, dir on, pulse clock.
    • To load
      • Put data on b.
      • Pulse ld
    • 
    • This is our previous circuit bundedled into one.
    • The 74161 is an example of this.
      • It is a bit more complex.
      • Clear, LD are all not lines, so they should be high
    • To increment the clock
      • ENP, ENT, LD and clr are high.
      • Pulse the clock.
    • To load from bus
      • Set everything but clr to low.
      • Put the data on the A-D
      • Pulse the clock.
    • 
    • This is the chip that Ben uses.
    • 
    • 
• We can build one more register related item, a register file.
  • We discussed this last time, but I wanted a circuit.
  • This one will allow four registers.
  • It uses an encoder and a mux.
  • 
• The digit file