Digital Counters
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.
Asynchronous or ripple counters
Synchronous counters.
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 Q_{A} output is applied to the clock input of the next flipflop i.e. FFB.
Logical Diagram
Operation
S.N.  Condition  Operation 

1  Initially let both the FFs be in the reset state  Q_{B}Q_{A} = 00................initially 
2  After 1st negative clock edge 
Q_{B}Q_{A} = 01................After the first clock pulse 
3  After 2nd negative clock edge 
Q_{B}Q_{A} = 10................After the second clock pulse 
4  After 3rd negative clock edge 
Q_{B}Q_{A} = 11................After the third clock pulse 
5  After 4th negative clock edge 
Q_{B}Q_{A} = 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 J_{A} and K_{A} inputs of FFA are tied to logic 1. So FFA will work as a toggle flipflop. The J_{B} and K_{B} inputs are connected to Q_{A}.
Logical Diagram
Operation
S.N.  Condition  Operation 

1  Initially let both the FFs be in the reset state  Q_{B}Q_{A} = 00................initially 
2  After 1st negative clock edge 
Q_{B}Q_{A} = 01................After the first clock pulse 
3  After 2nd negative clock edge 
Q_{B}Q_{A} = 10................After the second clock pulse 
4  After 3rd negative clock edge 
Q_{B}Q_{A} = 11................After the third clock pulse 
5  After 4th negative clock edge 
Q_{B}Q_{A} = 00................After the fourth clock pulse 
Classification of counters
Depending on the way in which the counting progresses, the synchronous or asynchronous counters are classified as follows.
Up counters
Down counters
Up/Down counters
UP/DOWN Counter
In the up/down counter, when up counter and down counter combined together to obtain an UP/DOWN counter. A mode control (M) input is also provided to select either up or down mode. A combinational circuit is required to be designed and used between each pair of flipflop in order to achieve the up/down operation.
Type of up/down counters
UP/DOWN ripple counters
UP/DOWN synchronous counters
UP/DOWN Ripple Counters
In the UP/DOWN ripple counter all the FFs operate in the toggle mode. So either T flipflops or JK flipflops are to be used. The LSB flipflop receives clock directly. But the clock to every other FF is obtained from (Q = Q bar) output of the previous FF.
UP counting mode (M=0)  The Q output of the preceding FF is connected to the clock of the next stage if up counting is to be achieved. For this mode, the mode select input M is at logic 0 (M=0).
DOWN counting mode (M=1)  If M =1, then the Q bar output of the preceding FF is connected to the next FF. This will operate the counter in the counting mode.
Example
3bit binary up/down ripple counter.
3bit : hence three FFs are required.
UP/DOWN : So a mode control input is essential.
For a ripple up counter, the Q output of preceding FF is connected to the clock input of the next one.
For a ripple up counter, the Q output of preceding FF is connected to the clock input of the next one.
For a ripple down counter, the Q bar output of preceding FF is connected to the clock input of the next one.
Let the selection of Q and Q bar output of the preceding FF be controlled by the mode control input M such that, If M = 0, UP counting. So connect Q to CLK. If M = 1, DOWN counting. So connect Q bar to CLK
Block Diagram
Truth Table
Operation
S.N.  Condition  Operation 

1  Case 1: With M = 0 (Up counting mode) 

2  Case 2: With M = 1 (Down counting mode) 

Modulus Counter (MODN Counter)
The 2bit ripple counter is called as MOD4 counter and 3bit ripple counter is called as MOD8 counter. So in general, an nbit ripple counter is called as moduloN counter. Where,MOD number = 2^{n}
Type of modulus
2bit up or down (MOD4)
3bit up or down (MOD8)
4bit up or down (MOD16)
Application of the counters
Frequency counters
Digital clock
Time measurement
A to D converter
Frequency divider circuits
Digital triangular wave generator