FlipFlops

# FlipFlops - Behavior Each flip-flop stores a single bit of...

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Behavior Each flip-flop stores a single bit of data, which is emitted through the Q output on the east side. Normally, the value can be controlled via the inputs to the west side. In particular, the value changes when the clock input, marked by a triangle on each flip-flop, rises from 0 to 1; on this rising edge, the value changes according to the corresponding table below. Another way of describing the different behavior of the flip-flops is in English text. D Flip-Flop: When the clock rises from 0 to 1, the value remembered by the flip-flop becomes the value of the D input ( Data ) at that instant. T Flip-Flop: When the clock rises from 0 to 1, the value remembered by the flip-flop either toggles or remains the same depending on whether the T input ( Toggle ) is 1 or 0. J-K Flip-Flop: When the clock rises from 0 to 1, the value remembered by the flip-flop toggles if the J and K inputs are both 1, remains the same if they are both 0, and changes to the K input value if J and K are not equal. (The names J and K do not stand for anything.) R-S Flip-Flop: When the clock rises from 0 to 1, the value remembered by the flip-flop remains unchanged if R and S are both 0, becomes 0 if the R input ( Reset ) is 1, and becomes 1 if the S input ( Set ) is 1. The behavior in unspecified if both inputs are 1. (In Logisim, the value in the flip-flop remains unchanged.) Truth Table

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The D flip-flop One of the main disadvantages of the basic SR NAND Gate bistable circuit is that the indeterminate input condition of "SET" = logic "0" and "RESET" = logic "0" is forbidden. This state will force both outputs to be at logic "1", over-riding the feedback latching action and whichever input goes to logic level "1" first will lose control, while the other input still at logic "0" controls the resulting state of the latch. In order to prevent this from happening an inverter can be connected between the "SET" and the "RESET" inputs to produce another type of flip- flop circuit called a Data Latch , Delay flip-flop , D-type Bistable or simply a D-type flip-flop as it is more generally called. The D flip-flop is by far the most important of the clocked flip-flops as it ensures that ensures that inputs S and R are never equal to one at the same time. D-type flip-flops are constructed from a gated SR flip-flop with an inverter added between the S and the R inputs to allow for a single D (data) input. This single data input D is used in place of the "set" signal, and the inverter is used to generate the complementary "reset" input thereby making a level-sensitive D-type flip- flop from a level-sensitive RS-latch as now S = D and R = not D as shown. D flip-flop Circuit We remember that a simple SR flip-flop requires two inputs, one to "SET" the output and one to "RESET" the output. By connecting an inverter (NOT gate) to the SR flip-flop we can "SET" and "RESET" the flip-flop using just one input as now the two input signals are complements of each other. This complement avoids the ambiguity inherent in the SR latch when both inputs are
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FlipFlops - Behavior Each flip-flop stores a single bit of...

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