A counter is a circuit that counts the number of occurrences of an input. The circuit consists of flip-flops which along with combinational elements are used for generation of control signals.

If M = Total number of states, and
   n = Total number of flip-flop
Then,      M <= 2n
If         M = 2n    ; Binary counter
and        M < 2n    ; Non-binary counter 

Block Diagram of Non-Binary Counter:
Here is the block diagram of a mod-10 counter. Here, the total number of flip-flops required is 4 thus the number of used state is 10 and number of unused state is 6.

In order to design a non binary counter a logic gate is required which detects M stage. In mod-10 coounter 10 stages are detected from 0000 to 10001 and as soon as 1010 appears it clears all the flip-flops.

Making of Non-binary counter:
For making non-binary counter,

• If CLR is present and CLK is connected with output Q, then we use AND gate.
• If CLR is present and CLK is connected with output Q’, then we use NOR gate.
• If complement of CLR is present and CLK is connected with output Q, then we use NAND gate.
• If complement of CLR is present and CLK is connected with output Q’, then we use OR gate.

Truth Table:

It is clear from truth table that as soon as 1010 appears, inputs to the AND gate becomes 11 which resets the flip-flop with CLR =1.

Output Frequency:
The output frequency of mod-M counter,

= f / M 

If there is no feedback present at last output (i.e D) then the output frequency,

= f / (2n) 

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