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The D Flip-Flop

The edge-triggered D flip-flop is easily derived from its RS counterpart. The only requirement is to replace the R input with an inverted version of the S input, which thereby becomes D. This is only needed in the master latch section; the slave remains unchanged.

One essential point about the D flip-flop is that when the clock input falls to logic 0 and the outputs can change state, the Q output always takes on the state of the D input at the moment of the clock edge. This was not true of the RS and JK flip-flops. The RS master section would repeatedly change states to match the input signals while the clock line is logic 1, and the Q output would reflect whichever input most recently received an active signal. The JK master section would receive and hold an input to tell it to change state, and never change that state until the next cycle of the clock. This behavior is not possible with a D flip-flop.

The edge-triggered D NAND flip-flop is shown below.

Edge-triggered D NAND flip-flop

With all of these different types of latches and flip-flops, the logic diagrams we have been using have gotten rather large, especially for the edge-triggered flip-flops. Fortunately, it really isn't necessary to follow and understand the inner workings of any of these circuits when they are used in larger applications. Instead, we use a set of very simple symbols to represent each type of latch or flip-flop in larger logical circuits. That is the subject of the next page on Flip-Flop Symbols.

Prev: D Latch Next: Flip-Flop Symbols

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