Sequential Circuits – Computer Science Notes – For W.B.C.S. Examination.

সিকোয়েন্সিয়াল সার্কিট – কম্পিউটার সায়েন্স নোট – WBCS পরীক্ষা।

A Sequential circuit combinational logic circuit that consists of inputs variable (X), logic gates (Computational circuit), and output variable (Z).Continue Reading Sequential Circuits – Computer Science Notes – For W.B.C.S. Examination.

Combinational cuit produces an output based on input variable only, but Sequential circuit produces an output based on current input and previous input variables. That means sequential circuits include memory elements which are capable of storing binary information. That binary information defines the state of the sequential circuit at that time. A latch capable of storing one bit of information.

1. External inputs which not controlled by the circuit.
2. Internal inputs which are a function of a previous output states.

Secondary inputs are state variables produced by the storage elements, where as secondary outputs are excitations for the storage elements.

Types of Sequential Circuits – There are two types of sequential circuit :

1. Asynchronous sequential circuit – These circuit do not use a clock signal but uses the pulses of the inputs. These circuits are faster than synchronous sequential circuits because there is clock pulse and change their state immediately when there is a change in the input signal. We use asynchronous sequential circuits when speed of operation is important and independent of internal clock pulse.But these circuits are more difficult to design and their output is uncertain.
2. Synchronous sequential circuit – These circuit uses clock signal and level inputs (or pulsed) (with restrictions on pulse width and circuit propagation). The output pulse is the same duration as the clock pulse for the clocked sequential circuits. Since they wait for the next clock pulse to arrive to perform the next operation, so these circuits are bit slower compared to asynchronous. Level output changes state at the start of an input pulse and remains in that until the next input or clock pulse.We use synchronous sequential circuit in synchronous counters, flip flops, and in the design of MOORE-MEALY state management machines.We use sequential circuits to design Counters, Registers, RAM, MOORE/MEALY Machine and other state retaining machines.
   

   Flip Flop

   Flip flop is a sequential circuit which generally samples its inputs and changes its outputs only at particular instants of time and not continuously. Flip flop is said to be edge sensitive or edge triggered rather than being level triggered like latches.

   S-R Flip Flop

   It is basically S-R latch using NAND gates with an additional enable input. It is also called as level triggered SR-FF. For this, circuit in output will take place if and only if the enable input (E) is made active. In short this circuit will operate as an S-R latch if E = 1 but there is no change in the output if E = 0.

   Operation

   S.N.	Condition	Operation
   1	S = R = 0 : No change	If S = R = 0 then output of NAND gates 3 and 4 are forced to become 1. Hence R’ and S’ both will be equal to 1. Since S’ and R’ are the input of the basic S-R latch using NAND gates, there will be no change in the state of outputs.
   2	S = 0, R = 1, E = 1	Since S = 0, output of NAND-3 i.e. R’ = 1 and E = 1 the output of NAND-4 i.e. S’ = 0. Hence Qn+1 = 0 and Qn+1 bar = 1. This is reset condition.
   3	S = 1, R = 0, E = 1	Output of NAND-3 i.e. R’ = 0 and output of NAND-4 i.e. S’ = 1. Hence output of S-R NAND latch is Qn+1 = 1 and Qn+1 bar = 0. This is the reset condition.
   4	S = 1, R = 1, E = 1	As S = 1, R = 1 and E = 1, the output of NAND gates 3 and 4 both are 0 i.e. S’ = R’ = 0. Hence the Race condition will occur in the basic NAND latch.

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