# What is state in a sequential circuit?

I understand that the output of a flip flop is also called as the state and the output of the flip flop before the clock pulse is applied is called as present state and after the application of clock pulse is called as next state. But what is state in a general sequential circuit, is it the output of the logic gate from which the output is taken as feedback?

The state of a general sequential circuit is no different to the state of the flip flop in your specific example (a flip flop is simply a state machine with two possible states). It shows the state the circuit is currently in.

The outputs of the various gates and circuits that feed the flip-flops determine the next state the machine will occupy, but until the transition actually takes place they are just provisional and may change at any time according to changes in inputs or other factors driving state changes.

A sequential circuit consists of combinationial logic elements (which are said to be stateless) and memory elements (which are stateful). The state of the circuit is the combined state of all the memory elements. If the memory elements are flip-flops, then the state of each flip-flop is the same as its output.

Note that this definition assumes the combinational logic part does not contain any loops - all loops need to contain a memory element.

'State' is one stable configuration a circuit can take.

In a general binary sequential circuit with M memory elements, there are $2^M$ states. It's possible that some of these could be transient given appropriate asynchronous gating, so that there could be fewer stable states.

A common way to handle the state of a sequential circuit is the 'state vector', a vector that simply lists the state of each memory element. That could for convenience be encoded into some other alphabet, but that's only convenient. For instance, a 4 bit shift register might have a state vector of 0110, but it could be easier to think of that as '6' when tracking what its state is, or where it's going to go next.

The state of a circuit is purely the collection of all of the current signals within that circuit. It is the status of the circuit.

For a purely sequential circuit the full state can be calculated purely from the inputs to the circuit.
For a simple circuit with memory such as a flipflop the state can be fully described by a combination of the inputs and the output.
For a complex circuit (e.g. a one with internal memory) knowing the state requires either knowledge of some of the internal signals or knowledge of the history of the inputs.