![]() ![]() Asynchronous counter circuit design is based on the fact that each bit toggle happens at the same time that the preceding bit toggles from a “high” to a “low” (from 1 to 0). Let’s examine the four-bit binary counting sequence again, and see if there are any other patterns that predict the toggling of a bit. However, if we simply connect all the J and K inputs to the positive rail of the power supply as we did in the asynchronous circuit, this would clearly not work because all the flip-flops would toggle at the same time: with each and every clock pulse! Now, the question is, what do we do with the J and K inputs? We know that we still have to maintain the same divide-by-two frequency pattern in order to count in a binary sequence, and that this pattern is best achieved utilizing the “toggle” mode of the flip-flop, so the fact that the J and K inputs must both be (at times) “high” is clear. 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 same clock pulse at the exact same time: The block diagram of this circuit is shown in Fig.3.A synchronous counter, in contrast to an asynchronous counter, is one whose output bits change state simultaneously, with no ripple. These circuits are difficult to design and are faster as compared to the synchronous circuits as the clock is not present. Latches are used as memory elements in these circuits. In these circuits, the output of the circuit can change state at any time, as soon as any input changes its state. 2 block diagram of a synchronous sequential circuit 2. The block diagram of this circuit is shown in Fig.2. These circuits are easy to design and they are slower. Flip flops are used as memory elements in these circuits. These are also called as clocked sequential circuits. ![]() In these circuits, a clock signal is used to determine/control the exact time at which any output can change its states. There are two types of sequential circuits, they are named as synchronous and asynchronous sequential circuits. ![]() There exists a feedback path between input and output.Įlementary building blocks are logic gates.Įlementary building blocks are flip flops.Įxamples: Adder, Encoder, Decoder, Multiplexer, etc.Įxamples: Flip flops, Counter, Register, etc. There is no feedback path between input and output. The circuit does not have a memory element. Output depends on the present input and past output. Output depends only on the present input. Difference between Combinational and Sequential circuits S.No. ![]() It simply means, in sequential circuits, the present output depends on the present input as well as on the past output(s). The binary information stored in these elements at any given time defines the state of the sequential circuits at that time. The storage/memory element is capable of storing binary information which is basically the past output. It consists of a combinational circuit to which storage or memory elements are connected to form a feedback path. A block diagram of a sequential circuit is shown in Fig.1. ![]()
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