Flip Flops

Flip Flops §

• Sequential vs Combinational logic
  • The combinational circuit does not use any memory
    • The previous state of input does not have any effect on the present state of the circuit. That is, the output is only dependent on the inputs at that time
  • But sequential circuit has memory can vary based on the previous input
    • This type of circuitry uses previous input, output and a memory element
• A flip-flop is memory
  • Flip-flops can be used to store a single bit. They can be used as a basis for memory
    • Flipflops are used in SRAM to hold state, rather than capacitors as used in DRAM
  • There are several types of flip-flop, but you need to know
    • Flip-flops have two outputs, Q and the inverse, Not(Q)
    • S-R Flip-Flop
      • NOR
        • 
        • The input lables are where the name is derived. S et, R eset
        • 
      • NAND
        • 
    • J-K Flip-Flop
      • The S-R Flip-flop’s invalid state is a problem. Another problem is metastability.
        • A single input being delayed could cause the output to change unexpectedly
        • Metastability means that a metastable state is similar to an unstable equilibrium. While in a metastable state, the outputs are unpredictable.
        • The J-K flip-flop over comes these by using an additional input: a clock, and removing the invalid state
          • The clock must be high (1) for the J and K inputs