The D Latch

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The D Latch

One very useful variation on the RS latch circuit is the Data latch, or D latch as it is generally called. As shown in the logic diagram below, the D latch is constructed by using the inverted S input as the R input signal. The single remaining input is designated "D" to distinguish its operation from other types of latches. It makes no difference that the R input signal is effectively clocked twice, since the CLK signal will either allow the signals to pass both gates or it will not.

For comparison, you can review the RS NAND latch circuit if you wish. Use the "Back" button or equivalent to return here.

In the D latch, when the CLK input is logic 1, the Q output will always reflect the logic level present at the D input, no matter how that changes. When the CLK input falls to logic 0, the last state of the D input is trapped and held in the latch, for use by whatever other circuits may need this signal.

Because the single D input is also inverted to provide the signal to reset the latch, this latch circuit cannot experience a "race" condition caused by all inputs being at logic 1 simultaneously. Therefore the D latch circuit can be safely used in any circuit.

Although the D latch does not have to be made edge triggered for safe operation, there are some applications where an edge-triggered D flip-flop is desirable. This can be accomplished by using a D latch circuit as the master section of an RS flip-flop, as shown on this page. Both types are useful, so both are made commercially available.

Except for the change in input circuitry, a D flip-flop works just like the RS 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 a separate page on Flip-Flop Symbols.