There are two main types of sequential circuits, and their classification is a function of the timing of their signals.Īsynchronous sequential circuit: A system whose behavior depends upon the input signals at any instant of time and the order Thus, a sequential circuit is specified by a time sequence of inputs, outputs, and internal states. The next state of the storage elements is also a function of external inputs and the present state. The block diagram demonstrates that the outputs in a sequential circuit are a function not only of the inputs, but also of the present state of the storage elements. These external inputs also determine the condition for changing the state in the storage elements. The sequential circuit receives binary valued inputs from external sources, this together with the present state of the storage elements determines the binary value of the outputs. This information that is stored at any time defines the state of the sequential circuit at that time. Storage elements are devices capable of storing binary information. Sequential Logic consists of combinational circuit to which storage elements are connected to form a feedback path. In a sequential circuit, everything is combinational except the memory and feedback.Although every digital system is likely to have some combinationalĬircuits, most systems encountered in practice also include storage elements which require that the system can be described in terms of sequential logic. So this will become past output and this past output will go to the input and it can be called as the feedback will go as the feedback and the present output will depend upon this present input and the past output. So as a result there is a present inputs and the outputs and also the memory element which has already stored the output from the previous inputs. Let’s say time comes and it’s time to use the values stored in this memory and these values will be used as the inputs. This memory will keep the output till the time the it will be going to be used again. You can see inputs and outputs on the diagram and also at the same time there is a storing process happening that particular output on outputs on the memory block. The present state and present inputs will define output and new state. The information stored in the sequential circuit represents the present state. That means past inputs are past output and the present state is nothing but past outputs. Here the present state depends upon past inputs only. In a sequential circuit, the present output depends on the present input as well as the present state(past output). It can have n number of inputs and m number of outputsĪ sequential circuit is generally a combinational circuit with memory.It doesn’t depend upon any previous input or output.Output is dependent upon the combination of input variables.So generally this is a logic circuit that has different logic gates integrated into its circuit. As the name indicates the output of a combinational circuit will depend only on combinational input variables given. The combinational logic circuit is a circuit whose output will depend only on the input variables. These two circuits are very important and broad categories of circuits defined in digital electronics where the combinational circuits are independent of time and the sequential circuits are dependent on time. This post answers the question “What is sequential and combinational logic?”.
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