Sequential Circuits

• Circuits with a memory.
• They all have some form of internal feedback.
• The Basic components
  • A tri-state device
    • A data input line
    • A data output line
    • A control line
    • If the control line is a 1, data is allowed to pass through.

    • din	c	dout
      x	0	nothing
      x	1	x

    • 
    • This is really useful for managing a bus.
  • Clock
    • Generates a regular series of pulses.
    • We may wish to delay a clock pulse
    • 
  • Many sequential circuits are "edge triggered"
    • This means they change state only on the leading edge of a clock pulse.
  • Other devices are "latch" devices
    • This means they change state on a "high" clock signal.
  • Flip Flops
    • A basic unit of memory.
    • Each has a setup time and a hold time
    • Setup time - the time the signal must be stable on the line before a state change can occur
    • Hold time - the time a signal must remain stable before the device changes state.
    • There are many different types,
    • RS, JK, D are all discussed in the book.
    • TkGate has a D, so we will study that first.
      • One data input (D)
      • Two data outputs (Q, Q')
      • a clock input (ck or ^)
      • An enable (not) control line
      • An clear (not) control line.
      • When the clock goes from low to high, the data on D is stored, and comes out at Q.
      • But only if the clear (not) line is 1
      • And the enable(not) line is 0
      • And sufficient time has gone by for the circuit to stabalize before the clock changed.
      • 
    • The RS FlipFlop
      • 
      • Two output lines, Q, Q' the state of the flipflop
      • Two control lines
        • S set - set Q to be 1
        • R reset - set Q to be 0
      • raising bot S and R is undefined.
    • Back to D
      • Add some extra logic, and we can create a D flipflop from an RS
      • 
      • The File
      • 
    • The JK Flipflop
      • Add additional logic to RS to eliminate strange can't raise both.
      • Two control lines - J,K
      • Two outputs
      • A clock.
      • J sets (Q = 1)
      • K resets (Q = 0)
      • JK flips.
• Some example circuits
  • A simple Register
    • n bits wide
    • A collection of n flipflops
    • A common clock line
    • An input line (n bits wide)
    • An output line (n bits wide)
    • A clear not line
    • An enable not line
    • 
    • Start by clearing the register
    • Change the value of the input
    • press the spacebar 10 times (5 will do)
    • Throw the clock switch
    • 
  • A Shift Register
    • We may want a line that will let us shift left or shift right
    • or a line that will let us load.
    • And a line that tells us what to fill with.
    • 
  • A memory
    • Data lines (n)
    • Address lines (m)
    • 1 read/ not write
    • 1 chip celect
    • 1 reset.
    • 
    • a file