Sequential Circuit  It is a type of logic circuit whose output depends not only on the present value of its input signals but on the past history of its inputs.  It is in contrast to combinational logic, whose output is a function of only the present input.  It has state (memory) while combinational logic does not. Or, in other words, sequential logic is combinational logic with memory.  It is used to construct:-  basic building block in all digital circuitry  memory circuits and other devices

 Digital sequential logic circuits are divided into synchronous and asynchronous types.  In synchronous sequential circuits, the state of the device changes only at discrete times in response to a clock signal.  In asynchronous circuits the state of the device can change at any time in response to changing inputs.

Control of an alarm system  the simplest case of a sequential circuit  Alarm is on when the sensor generates the “Set” signal in response to some undesirable events  Once the alarm is on, it can only be turned off manually through a reset button  Memory is needed to remember that the alarm has to be active until the reset signal arrives

Synchronous Sequential Circuits  All sequential logic today is clocked or synchronous logic.  In a synchronous circuit, a clock (or clock generator) generates a sequence of the clock signal which is distributed to all the memory elements in the circuit.  The basic memory element in sequential logic is the flip-flop.  The output of all the storage elements (flip-flops) in the circuit at any given time, the binary data they contain, is called the state of the circuit.  The state of a synchronous circuit only changes on clock pulses.

fig. Synchronous circuit

Asynchronous Sequential Circuits  Asynchronous sequential logic is not synchronized by a clock signal; the outputs of the circuit change directly in response to changes in inputs.  The advantage of asynchronous logic is that it can be faster than synchronous logic, because the circuit doesn't have to wait for a clock signal to process inputs.  The speed of the device is potentially limited only by the propagation delays of the logic gates used.  However, asynchronous logic is more difficult to design and is subject to problems not encountered in synchronous designs.

 The main problem is that digital memory elements are sensitive to the order that their input signals arrive; if two signals arrive at a logic gate at almost the same time, which state the circuit goes into can depend on which signal gets to the gate first.  Therefore the circuit can go into the wrong state, depending on small differences in the propagation delays of the logic gates. This is called a race condition.

fig. asynchronous circuits

Counter  In digital logic and computing, a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal.  Counters can be implemented quite easily using register-type circuits such as the flip-flop, and a wide variety of classifications exist:  Asynchronous (ripple) counter – changing state bits are used as clocks to subsequent state flip-flops  Synchronous counter – all state bits change under control of a single clock  Decade counter – counts through ten states per stage  Up/down counter – counts both up and down, under command of a control input  Ring counter – formed by a shift register with feedback connection in a ring  Johnson counter – a twisted ring counter  Cascaded counter  modulus counter.

1) Asynchronous (ripple) counter:- An asynchronous (ripple) counter is a single d-type flip-flop, with its J (data) input fed from its own inverted output. This circuit can store one bit, and hence can count from zero to one before it overflows (starts over from 0). 2) Synchronous counter:- In synchronous counters, the clock inputs of all the flip-flops are connected together and are triggered by the input pulses. Thus, all the flip-flops change state simultaneously (in parallel).

Register  Registers are groups of flip-flops, where each flip- flop is capable of storing one bit of information.  An n-bit register is a group of n flip-flops.  The basic function of a register is to hold information in a digital system and make it available to the logic elements for the computing process.  Since each flip-flop is capable of storing either a "0" or a "1", there is a finite number of 0-1 combinations that can be stored into a register.

 Each of those combinations is known as state or content of the register.  With flip-flops we can store data bitwise but usually data does not appear as single bits.  Instead it is common to store data words of n bit with typical word lengths of 4, 8, 16, 32 or 64 bit.

Shift Registers :-  A shift register is an n-bit register with a provision for shifting stored data by one bit position at each tick of the clock.

Shift Registers – Serial-in, Parallel-out

Sequential Circuit Design and Procedure 1. Problem Statement 2. State Table 3. The number of States May be reduced 4. Assign binary variable to each state 5. Determine number of flip-flop and assign a letter symbol to each 6. Choose the type of flip-flop to be used 7. From the state table, derived the circuit excitation and output tables 8. Simplify 9. Draw the Logic diagram

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