A sequential circuit is a combinational circuit with some feedback from the outputs. In a sequential circuit, the output state depends on both the inputs and the outputs. The term “sequential” comes from the fact that the output depends not only on the current states, but on the states immediately preceding. A flipflop is usually shown as a rectangle with two or more inputs and two outputs. FLIPFLOPThe most important and basic type of a sequential circuit is a FLIPFLOP. There are 4 kinds of FlipFlops namely— • RS flipflop • D flipflop • T flipflop • JK flipflop RS flipflopThe inputs of an RS flipflop are labeled R (reset) and S (set) and the outputs are labeled as Q and Q’.D flipflopA D flipflop has only one input (D) and two outputs (Q and Q’). In a D flipflop, the output Q sets its value similar to the value of the input D.T flipflopA T flipflop has only one input (T) and two outputs (Q and Q’). In a T flipflop, the output Q toggles its value depending on the value of the input T.JK FlipFlopThe JK flipflop is the most versatile of the basic flipflops. It has two inputs, traditionally labeled J and K. If J and K are different then the output Q takes the value of J at the next clock edge.Counters / TimersA timer or counter is a series of bistables or flipflops that change state once for every input signal, thus one of these circuits would divide the input frequency by a factor of two. If this signal is then fed into the next bistable, the output is 1/4 of the original frequency. The next circuit would have an output of 1/8, then 1/16 and so on. Counters exploit the toggling behavior of JK flipflops which is observed when J and K are set to 1. In counters, the JK flipflops are arranged one after the other feeding at the output of the preceding flipflops. Counters can be either Asynchronous or Synchronous. An Example of Asynchronous Counters : Ripple CountersIn a ripple counter, the flipflop output transition serves as a source for triggering other flipflops. In other words, the CP inputs of all flipflops (except the first) are triggered not by the incoming pulses, but rather by the transition that occurs in other flipflops. An Example of Synchronous Counters : Binary CounterIn a synchronous counter, the input pulses are applied to CP inputs of all flipflops. The change of state of a particular flipflop is dependent on the present state of other flipflops. The common pulse triggers all the flipflops simultaneously, rather than one at a time in succession as in a ripple counter. The decision whether a flipflop is to be complemented or not is determined from the values of the J and K inputs at the time of the pulse. If J = K = 0, the flipflop remains unchanged. If J = K = 1, the flipflop complements. The Binary Counter  In a synchronous binary counter, the flipflop in the lowestorder position is complemented with every pulse. This means that its J and K inputs must be maintained at logic1. A flipflop in any other position is complemented with a pulse provided all the bits in the lowerorder positions are equal to 1, because the lowerorder bits (when all 1s) will change to Os on the next count pulse. The binary count dictates that the next higherorder bit be complemented. These counters have been covered in a reasonably detailed manner in this tutorial document Complete Tutorial document with truthtables, examples and examples of circuits :Here's a list of all the tutorials we currently have in this area  Introductory Digital Electronic Circuits and Boolean logic
