Tag: Flip Flops
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11.5 Finite State Machines
Up to now, every circuit that was presented was a combinatorial circuit. That means that its output is dependent only by its current inputs. Previous inputs for that type of circuits have no effect on the output. However, there are many applications where there is a need for our circuits to have “memory”; to remember…
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11.3 Synchronous Counters
What is a Synchronous Counter? A synchronous counter, in contrast to an asynchronous counter, is one whose output bits change state simultaneously, with no ripple. The only way we can build such a counter circuit from J-K flip-flops is to connect all the clock inputs together, so that each and every flip-flop receives the exact…
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11.2 Asynchronous Counters
In the previous section, we saw a circuit using one J-K flip-flop that counted backward in a two-bit binary sequence, from 11 to 10 to 01 to 00. Since it would be desirable to have a circuit that could count forward and not just backward, it would be worthwhile to examine a forward count sequence…
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11.1 Binary Count Sequence
If we examine a four-bit binary count sequence from 0000 to 1111, a definite pattern will be evident in the “oscillations” of the bits between 0 and 1: Note how the least significant bit (LSB) toggles between 0 and 1 for every step in the count sequence, while each succeeding bit toggles at one-half the…
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10.8 Monostable Multivibrators
We’ve already seen one example of a monostable multivibrator in use: the pulse detector used within the circuitry of flip-flops, to enable the latch portion for a brief time when the clock input signal transitions from either low to high or high to low. The pulse detector is classified as a monostable multivibrator because it…
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10.7 Asynchronous Flip-Flop Inputs
The normal data inputs to a flip flop (D, S and R, or J and K) are referred to as synchronous inputs because they have effect on the outputs (Q and not-Q) only in step, or in sync, with the clock signal transitions. These extra inputs that I now bring to your attention are called…
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10.6 The J-K Flip-Flop
Another variation on a theme of bistable multivibrators is the J-K flip-flop. Essentially, this is a modified version of an S-R flip-flop with no “invalid” or “illegal” output state. Look closely at the following diagram to see how this is accomplished: The J and K Inputs What used to be the S and R inputs…
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10.5 Edge-triggered Latches: Flip-Flops
So far, we’ve studied both S-R and D latch circuits with enable inputs. The latch responds to the data inputs (S-R or D) only when the enable input is activated. In many digital applications, however, it is desirable to limit the responsiveness of a latch circuit to a very short period of time instead of…
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10.4 The D Latch
Since the enable input on a gated S-R latch provides a way to latch the Q and not-Q outputs without regard to the status of S or R, we can eliminate one of those inputs to create a multivibrator latch circuit with no “illegal” input states. Such a circuit is called a D latch, and…
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10.3 The Gated S-R Latch
It is sometimes useful in logic circuits to have a multivibrator which changes state only when certain conditions are met, regardless of its S and R input states. The conditional input is called the enable, and is symbolized by the letter E. Study the following example to see how this works: When the E=0, the…
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10.2 The S-R Latch
A bistable multivibrator has two stable states, as indicated by the prefix bi in its name. Typically, one state is referred to as set and the other as reset. The simplest bistable device, therefore, is known as a set-reset, or S-R, latch. To create an S-R latch, we can wire two NOR gates in such…
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10.1 Digital Logic With Feedback
With simple gate and combinational logic circuits, there is a definite output state for any given input state. Take the truth table of an OR gate, for instance: For each of the four possible combinations of input states (0-0, 0-1, 1-0, and 1-1), there is one, definite, unambiguous output state. Whether we’re dealing with a…