Latches and flip-flops are the basic elements for storing information. Using the signal Q as the state variable to describe the state of the circuit, we can say that the circuit has two stable states: Q= 0, and Q'= 1 hence the name “bistable.” Hence we can say that the bistable circuit can store one bit information in it so called as one bit memory cell. A bistable element has memory in the sense that it can remember the state of the circuit indefinitely. Similarly, if we start the circuit with Q= 1, we will get Q' = 0, and again we get a stable situation. A 1 going to the input of the top inverter will produce a 0 at the output Q, which is what we started off with.
Since Q is also the input to the bottom inverter, Q', therefore, is a 1. However, Q will take on whatever value it happens to be when the circuit is first powered up.Īssume that Q = 0 when we switch on the power. Since the circuit has no inputs, we cannot change the values of Q and Q'. It has no inputs and two outputs labeled Q and Q'. The simplest sequential circuit or storage element is a bistable multivibrator, which is constructed with two inverters connected sequentially in a loop as shown in Figure below. The storage capability in sequential circuits is normally achieved by means of flip-flops. In this case, the system can be modeled as in Figure (b), where a feedback loop, containing the storage elements, can be observed. In contrast, a sequential logic circuit is one in which the outputs do depend on previous system states, so storage elements are necessary, as well as a clock signal that is responsible for controlling the system evolution. Thus the system is memory less and has no feedback loops, as in the model of Figure (a) below. Definition: A sequential logic circuit is one whose outputs depend not only on its current inputs, but also on the past sequence of inputs.Ī combinational logic circuit is one in which the outputs depend solely on the current inputs.
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