Digital Sequential Circuits
Counter is a sequential circuit. A digital circuit which is used for a counting pulses is known counter. Counter is the widest application of flipflops. It is a group of flipflops with a clock signal applied. Counters are of two types.
Table of contents
Asynchronous or ripple counters
The logic diagram of a 2bit ripple up counter is shown in figure. The toggle (T) flipflop are being used. But we can use the JK flipflop also with J and K connected permanently to logic 1. External clock is applied to the clock input of flipflop A and QA output is applied to the clock input of the next flipflop i.e. FFB.
Logical Diagram
Types
It is known as ripple counter because of the way the clock pulse ripples its way through the flipflops. The flipflop applied with external clock pulse act as LSB (Least Significant Bit) in the counting sequence.The flipflop toggles the output either for every positive edge of clock signal or for negative edge of clock signal.
 2 bit ripple up counter: It contains two flip flops. A 2bit ripple counter can count up to 4 states. It counts from 0 to 3.
 2 bit ripple down counter: It contains two flip flops. A 2bit ripple counter can count up to 4 states. It is known as down counter as it counts down from 3 to 0.

3 bit ripple up counter: It contains three flip flops. A 3bit ripple counter can count up to 8 states. It counts from 0 to 7.

3 bit ripple down counter: It contains three flip flops. A 3bit ripple counter can count up to 8 states.It counts down from 7 to 0.
 Decade or BCD counter : A binary coded decimal (BCD) is a serial digital counter that counts ten digits. It counts from 0 to 9.When the clock pulse advances to 10 the ports QB and QD become high and thus NAND gate’s output will become low which will reset all the flip flops.
Operation
Condition  Operation 

Initially let both the FFs be in the reset state  QBQA = 00 initially. 
After 1st negative clock edge  As soon as the first negative clock edge is applied, FFA will toggle and QA will be equal to 1.QA is connected to clock input of FFB. Since QA has changed from 0 to 1, it is treated as the positive clock edge by FFB. There is no change in QB because FFB is a negative edge triggered FF.QBQA = 01 after the first clock pulse. 
After 2nd negative clock edge  On the arrival of second negative clock edge, FFA toggles again and QA = 0.The change in QA acts as a negative clock edge for FFB. So it will also toggle, and QB will be 1.QBQA = 10 after the second clock pulse. 
After 3rd negative clock edge  On the arrival of 3rd negative clock edge, FFA toggles again and QA become 1 from 0.Since this is a positive going change, FFB does not respond to it and remains inactive. So QB does not change and continues to be equal to 1.QBQA = 11 after the third clock pulse. 
After 4th negative clock edge  On the arrival of 4th negative clock edge, FFA toggles again and QA becomes 1 from 0.This negative change in QA acts as clock pulse for FFB. Hence it toggles to change QB from 1 to 0.QBQA = 00 after the fourth clock pulse. 
Truth Table
Synchronous counters
If the “clock” pulses are applied to all the flipflops in a counter simultaneously, then such a counter is called as synchronous counter.
2bit Synchronous up counter
The JA and KA inputs of FFA are tied to logic 1. So FFA will work as a toggle flipflop. The JB and KB inputs are connected to QA.
Types
 4 bit synchronous up counter: In the up counter the 4 bit binary sequence starts from 0000 and increments up to 1111, i.e from 0 to 15.
 4 bit synchronous down counter: Down counter counts the numbers in decreasing order i.e from 15 to 0.
 4 bit synchronous up/down counter: This counter has two modes of counting i.e. up counting and down counting. There is a mode switch which switches between the two modes of the counter. When the mode M = 0 it counts up & when mode M = 1 then it counts down.
 4Bit Synchronous Decade Counter: A BCD decade counter exhibits a truncated binary sequence and goes from 0000 through the 1001 state. Rather than going from the 1001 state to the 1010 state, it recycles to the 0000 state.
 Ring Counter: The ring counter is a application of shift register, in which the output of last flip flop is connected to input of first flip flop. In ring counter if the output of any flip flop is 1, then the output of remaining flip flops is 0. The Ring counters transfers the same output throughout the circuit.
 Johnson’s Counter or Twisted Ring Counter: The Johnson counter is a modification of ring counter. In this the inverted output of the last stage flip flop is connected to the input of first flip flop. If we use n flip flops to design the Johnson counter, it is known as 2n bit Johnson counter or Mod 2n Johnson counter.
Logical Diagram
Operation
Condition  Operation 

Initially let both the FFs be in the reset state  QBQA = 00 initially. 
After 1st negative clock edge  As soon as the first negative clock edge is applied, FFA will toggle and QA will change from 0 to 1.But at the instant of application of negative clock edge, QA , JB = KB = 0. Hence FFB will not change its state. So QB will remain 0.QBQA = 01 after the first clock pulse. 
After 2nd negative clock edge  On the arrival of second negative clock edge, FFA toggles again and QA changes from 1 to 0.But at this instant QA was 1. So JB = KB= 1 and FFB will toggle. Hence QB changes from 0 to 1.QBQA = 10 after the second clock pulse. 
After 3rd negative clock edge  On application of the third falling clock edge, FFA will toggle from 0 to 1 but there is no change of state for FFB.QBQA = 11 after the third clock pulse. 
After 4th negative clock edge  On application of the next clock pulse, QA will change from 1 to 0 as QB will also change from 1 to 0.QBQA = 00 after the fourth clock pulse. 