D-flops

One of the most common kinds of flip-flops (or, just flops) is the D-type flop. Like all flops, it has the ability to remember one bit of digital information. What makes the D-flop special is that it is a clocked flip-flop. We'll spend some time looking at what that means.

First, let's go through the pins of a standard D-flop. The diagram above is for half of a 74HCT74 chip, which comes with two D-flops on one IC.

  • D is the Data Input pin. This is where the flop gets its information from.

  • Q is the Output pin. It shows what value the flop is currently remembering.

  • Q' is the Inverted Output pin. It shows the opposite of the value that the flop is remembering.

  • CLK is the clock pin. When a new clock pulse comes in, the flop checks the input pin D, and sets itself up to remember that input value. The D-flop is edge-triggere, which means that it responds to the rising edge of the clock pulse. This will be clearer when we look at a timing diagram, below.

  • R is an Active-LOW Reset pin. When the Reset pin gets a LOW signal, it resets the flop to remember a 0, or LOW value.

  • S (also called PRE on some diagrams) is an Active-Low Set pin. When it gets a LOW signal, it sets the flop to remember a 1, or HIGH value.

The flip-flop is the foundation of sequential logic. To understand how to use flops, we need to see how they function over time in reponse to different signals. We'll walk through a sequence, using a timing diagram to help show what's going on.

  • At the beginning of the diagram, the inputs to the D and CLK pins are LOW, and the Q output also happens to be LOW. At point A, the D input goes HIGH. However, since there is no clock pulse, the Q output does not change yet.

  • At point B, the CLK input sees a rising edge. That rising edge tells the flop to remember the D input.

  • A very short time passes from point B to point C, which is the time needed to actually make the flop remember the new value. Note that now the Q output is the same as the D input.

  • At point D, the D-input goes to LOW. However, the Q output does not change, because there is no clock pulse.

  • At point E, we have a falling edge on the CLK input. However, our flop only responds to rising edges, so the output Q still persists.

Note that the S and R pins will Set or Reset the flop regardless of the D input or clock pulses. These pins are often used to set the flop to an initial state before our logic circuits begin their work.

Here's the pinout of the 74HCT74N, a dual D-type flip-flop. Note that the 'dual' designation means that two flops come on one chip.


 

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