Sequential Circuits

 

A sequential circuit is a digital circuit whose output not only depends on  the present inputs but also on previous inputs and outputs. For sequential circuits, the values of variables are usually specified at certain discrete time instants rather than whole continuous time. Their general function is to store information. Sequential Circuits are consists of combinational circuits and memory elements are connected to it as feedback path. A block diagram of sequential circuit is below:

Characteristics of Sequential Circuits

• Sequential circuits consists of feedback path and several memory elements.
• State - The state of the memory devices now, also called current state. Next states and outputs are functions of inputs and present states of storage elements 
• In this output depends upon present as well as past input. 
• Speed is slow.
• It is difficult to design as compared to combinational circuits. Hence, it is not easy to use and handle.
• It is time dependent. 
• Elementary building blocks are latches and flip-flops
• Mainly used for storing data. 
• They have capability to store any state or to retain earlier state. 
• Sequential circuits are clock dependent. Hence, they need triggering. 

Difference b/w Combinational and Sequential Circuit

Types of Sequential Circuits

There are two major types of Sequential Circuits: Synchronous and Asynchronous Sequential Circuits.  

Synchronous Circuits 

In synchronous circuits, output changes at discrete interval of time. It is a circuit based on an equal state time or an state time depending on external means such as clock. 

Thus synchronous circuits can be divided into clocked and un-clocked or pulsed sequential circuits. Before discussing these two, first let us know what is clock. 

A clock signal is a signal used to coordinate the actions of two or more circuits. It oscillates between a high and a low state, and generally it is a square wave. Circuits that use the clock signal for synchronization may become active at either the rising or falling edge of the clock signal.

Clocked Sequential Circuit

The clocked sequential circuits have flip-flops or gated latches for its memory elements. There is a periodic clock connected to the clock inputs of all the memory elements of the circuit to synchronize all the internal changes of state. Hence the operation of the circuit is controlled and synchronized by the periodic pulse of the clock.

Unclocked Sequential Circuit 

An unclocked sequential circuit requires two consecutive transitions between 0 and 1 to alternate the state of the circuit. An unclocked mode circuit is designed to respond to pulses of certain durations which do not affect the circuit’s behaviour.

The synchronous logic circuit is very simple. The logic gates which perform the operations on the data, require a finite amount of time to respond to the changes in the input

Asynchronous Circuits 

In an asynchronous circuit, output can be changed at any interval of time by changing the output. Asynchronous circuit is a circuit whose state time depends solely upon internal logic circuit delays. It does not have a clock signal to synchronize its internal changes of the state. 

Asynchronous logic is more difficult to design and it has some problems compared to synchronous logic. The main problem is that the digital memory is sensitive to the order that their input signals arrive them, like, if two signals arrive at a flip-flop at the same time, which state the circuit goes into can depend on which signal gets to the logic gate first. Asynchronous circuits are used in critical parts of synchronous systems where the speed of the system is a priority, like as in microprocessors and digital signal processing circuits.

Applications of Sequential Circuits

The main applications of sequential logic circuits are:

• Latches and Flip flops: These are building block of sequential circuits. They can store 1-bit of data. 
• Shift registers: Group of flip flops used to store multiple bits data and also capable of transferring data.
• Counters: Collection of flip flops, with an applied clock signal, which is used to count the pulse.
• Analog to Digital and Digital to Analog Converters
• Used in programmable devices such as CPLD, PLD, and FPGA
• Also used as registers for microprocessors and controllers