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Computer organization Notes-Sequential Circuits

Sequential Circuits Sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs, the input history. Sequential circuit has memory so output can vary based on input. This type of circuits uses previous input, output, clock and a memory element. A sequential circuit is an interconnection of flip-flops and gates. The gates by themselves constitute a combinational circuit, but when included with the flip-flops, the overall circuit is classified as a sequential circuit. A sequential circuit is specified by time sequence of inputs, outputs and internal states. The binary information stored in the memory elements at any given point defines the state of sequential circuit and the current input as well as present state of memory elements determine the output of sequential circuit. The Block diagram of a sequential circuit is as shown below:

It consits of combinational circuit and a number of clocked flip-flops. We can have any number or any type of flip-flop. These combinational circuit receive external signals from external inputs and also from the output of flip-flop. The output of combinational circuit goes to external outputs and to the input of flip-flops. The gates in the combinational circuit determine the binary value to be stored in the flip-flop/memory unit after each clock transition. The outputs of flip-flop, in turns, are applied to the combinational circuit input and determine the circuit behaviour. In sequential circuit memory elements are connected to combinational circuits to form a feedback path. There are two types of sequential circuit.
1. Asynchronous Sequential Circuit
2. Synchronous Sequential Circuit

Asynchronous Sequential Circuit
Asynchronous sequential circuits are those sequential circuits in which the inputs and outputs do not change at preassigned times since the inherent delays are not rigidly controlled. The outputs of asynchromous sequential circuit depends upon the order in which its input variable changes and can be affected at any instant of time. Asynchoronous sequential circuits change their output states with no external synchronization. The flip-flop used, as memory element is time delayed in these circuits. The asynchronous sequential circuits are the combinational circuits with feedback. Feedback from memory element is provided to logocal gates. Hence these circuits become unstable due to transient conditions. The instability in the output state of these circuits impose many difficulties. This is why these circuits are not used commonly.
Synchronous Sequential Circuit
This type of circuits uses Flip-flops to change their binary values only at aa discrete instants of time. Synchronous sequential circuits uses logic gates and flip-flop storage devices. All state transitions in such circuit only when the clock value is either 1 or 0 i.e state transition occur at the rising or falling edges of the clock depending on the type of memory elements used in the circuit. A timing device called a clock pulse generator achieves synchronization. Clock pulses are distributed throughout the system in such a way that the flip-flops are affected only with the arrival of the synchronization pulse. They are stable and their timing can easily be broken down into independent discrete steps and each step is considered separately. Synchronous circuits are more common than the asuchronous circuits.

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