Synchronous sequential logic

Almost all the sequential logic today is ‘synchronous' or' clocked' logic: there is a 'clock' signal, and all the internal memory ('internal state') changes only on the clock edge. The basic storage element in the sequential logic is flip-flop.

The basic advantage of synchronous logic is the simplicity of it. Every operation in circuit should be completed inside the fixed interval of time in between 2 clock pulses, known as 'clock cycle'. As long as this condition is met (ignoring certain other details), the circuit is ensured to be reliable. Synchronous logic has 2 main disadvantages, which are stated as follows.

1. The clock signal should be distributed to every flip-flop in circuit. As the clock is generally a high-frequency signal, this distribution consumes a relatively large amount of power and dissipates much heat. Even the flip-flops that are doing nothing uses a small amount of power, therefore generating waste heat inside the chip.

2. The maximum clock rate which is possible is determined by slowest logic path in the circuit, otherwise called as the critical path. This means that every logical calculation, from easiest to most complexes, should complete in one clock cycle. One way around this restriction is to split complex operations into a number of simple operations, a method called as 'pipelining'. This method is prominent within microprocessor design, and helps to improve performance of the modern processors.

In the digital circuit theory, sequential logic is a logic circuit the output of which depends not only on present input but also on history of the input. This is in the contrast to combinational logic, the output of which is a function only of the present input. Or we can say that sequential logic has storage while the combinational logic does not.

Sequential logic is thus used to construct types of delay, some types of computer memory, and storage elements, and finite state machines. The most practical computer circuits are a combination of the combinational and sequential logic.

There are 2 types of finite state machine which can be built from the sequential logic circuits:

• Moore machine: the output depends on the internal state only. (Since the internal state changes on a clock edge only, the output changes on a clock edge too).
• Mealy machine: the output is dependent not only on the internal state, but also on the inputs.

Depending upon the regulations of functioning, digital circuits can be divided into synchronous and asynchronous. According to the behavior devices obeys synchronous or asynchronous logic.

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