Latches

Notes

• Carpinelli chapter 6.2
• A latch is a device that maintains its state after being activated and changes whenever the input is changed.
• A flip-flop is a device that maintains its state after being activated but changes only on a clock change.
• Flip-flops are built from latches.
• We will begin with latches
• The R-S or SR latch is the simplest.
  • Input:
    • S set the circuit
    • R reset the circuit
    • NOTE: We do not expect both set and reset to be high at the same time.
  • Output
    • Q and Q. The state of the latch.
• There are several implementation of a rs-latch
  • 
  • The difference is the action when R=1 and S=1
  • For the nor gate versions it is 0
  • For the nand gate versions it is 1.
• These are modified with an enable line
  • Just add another input, enable
  • And this with S and R respectively
  • Note, this is still not edge triggered.
  • 
• Note: I think rs/sr latch/flip-flop are somewhat interchangeable.
• Frequently we don't care about set or reset, we want to store data
  • This is the D-latch.
  • One input: D the data
  • Outputs Q and Q
  • Actually it is a simple modification to the rs-latch
  • 
  • This is not completely satisfying as when the data on d changes, the latch changes.
  • So add an enable line.
  • 
• There are other latches, but these two will do.
• My digital file