Latches and Flip Flops - University of Houston

Latches and Flip Flops - University of Houston

Latches and Flip Flops Latches A latch is a bistable device, with inputs, that remains in a given state as long as

power is applied and until input signals are applied to cause its output to change. A bistable device is a circuit having two stable conditions (states).

SR (set-Reset) Latch Next State Q or Q(t) denotes the present state of a memory device, i.e., the state at the time

the input signals are applied, Q+ or Q(t+1) denotes the next state, i.e., the new state assumed by the device in response to the input signals.

Gated SR Latch R Q

C Q' S Gated D Latch

Timing considerations Propagation delays Minimum pulse width

Setup and hold time Setup and hold times To achieve a satisfactory operation of a gated latch, constraints are normally

placed on the time intervals between input changes.The minimum time the input signal must be held fixed before and after the latching action is called the setuptime and holdtime, respectively.

Minimum pulse width The minimum amount of time a signal must be applied in order to produce a desired result.

Latches vs. Flip Flops Latches are flip-flops for which the timing of the output changes are not controlled. For a latch, the output responds

immediately to changes on the input lines. i.e the timing of output changes are not controlled. A flip-flop is designed to change its output at the edge of a controlling clock signal.

Positive/Negative Edge Triggering The transition of a control signal (clock pulse) from its low to high value (0 to 1) in positive logic is called the positive edge of

the control signal, while the transition from high to low (1 to 0) is called the negative edge. flip-flops are designed to change their output at the either the positive or negative

edge of a clock signal. Edge triggered Flip Flops Edge triggered flip-flops use just one of the edges of the clock pulse to affect the

reading of the input lines.These flip-flops are designed to be triggered by either the positive or negative edge.In analyzing the behavior of an asynchronous sequential circuit, one often needs to know which

edge trigger the flip-flops used. Flip Flops There are four different types of flip-flops: SR, D, JK, and T types.

JK Flip Flop A JK flip-flop works just like an SR flip-flop if we consider J input as S(et) input and K input as R(eset) input, except when both S

and R inputs are set to 1, the output simply flips over. JK Flip Flops

T (toggle) Flip Flop A T flip-flop is obtained from a JK flipflop by tying the J and K inputs together to form the T inpu

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