Make your own free website on
Clocked Logic:How counters work... (LESSON 4)

The D Flip-Flop

    A type of D-Flip Flop

    A D Flip-Flop ( D standing for Data or Delay ) can be made from a JK Flip-Flop if K is always the compliment of J. By adding an inverter between the two, this insures that J = input D and K = the compliment of input D. To simplify this circuit consider the figure below:

    In this representation, the JK Flip-Flop has been replaced by a square with all of the inputs and outputs of the JK Flip-Flop symbolized as letters. At this point you may be wondering, "what does the D Flip-Flop do?..." Primarily it is used as a data storage unit. It can store a bit of information similar to the RS Flip-Flop, but with a lot more capabity. Also, the D Flip-Flop can be cascaded to create shift registers. On the other hand, the D Flip-Flop can be made to act like a T Flip-Flop, which is somewhat of a backwards compatibility advantage, however it lacks some benefits that the JK Flip Flop has, such as it cannot be put into a non operational state, since the "internal" J and K inputs are coupled by an inverter. In order to achieve the "do-nothing" state, J and K both have to be low.

    An important, but possibly confusing concept, is the difference between level and edge clocked systems. Up to this point all of the flip flops have been level clocked based. They do something during one state and do something else on a different state. Unfortantely this seemingly error/glitch proof design has a considerable disadvantage. It cannot be used to reliably take "random" inputs from an external source. Imagine you have just built a frequency counter, dont worry you don't actually have to know how to build one of these yet, and it displays on its screen the number of pulses that come into its input per second. These inputs per second (also known as frequency: measured in Hertz Hz)  are essentially random . They are not synchronyzed with the frequency counter's internal clock. Since level clocked circuits can only take inputs at certain clock levels, there is no guarantee that every pulse received will actually be counted. If the inputs are not counted properly, the accuracy of the system is lost and it becomes unuseful.
    The solution to this problem is to design the Flip-Flops to be edge clocked. In an edge clocked system, Everything happens on the edge of the clock.
    In the level clocked T Flip-Flop from the previous section, information was passed into the master stage on one level and then passed to the slave stage, where it reaches the output, on the other clock level. In an edge clocked system, the information is transferred and displayed on the outputs on one particular clock level. This particular level depends on whether on not the design is positive or negative edge triggered. For this reason, an edge triggered device can have its inputs connected to a randomly changing source, but will only take these inputs in on a certain "edge" of the clock signal. A level clocked device must have its inputs wither synchronized with the clock or unchanging to work properly. The concept can be quite confusing, but will generally become easier to understand as you become more familiar with the application of flip-flops. The simulation ahead will deomonstarte the differences between a level clocked D Flip-Flop and an edge clocked D Flip-Flop.
    As a general rule, level clocked systems can be used to make clocks, counters, and sequencers, whereas edge clocked systems can be used to make synchronyzers (when accurately taking random data is important )  as well as the stuff level clocked circuits can make. It really depends on the application.
    Although both level and edge clocked D Flip-Flops exist, for most applications the edge clocked style would be preferable.  The following schematics are those of the level and edge clocked D Flip-Flops...

    The Negative Level Clocked D Flip-Flop (using NAND gates),  (Shown before at top of page)....
Level Clocked D Flip-Flop

    The Negative Edge Clocked D Flip-Flop (using NAND gates)....

Edge Clocked D Flip-Flop


Clocked Logic: Lesson 3
Clocked Logic: Lesson 4
  Clocked Logic: Lesson 5