A special place among counters has the mod -10 counter. The mod -10 counter passes through 10 states before he starts anew and thus it is interesting when the decimal number system is used...

In a **mod-10** counter you are using a
**4-bit** counter that actually has 8 states.
However, when the counter reaches the seventh state
(**Q3 = 1**,
**Q2 = 0**,
**Q1 = 1**,
**Q0 = 0**),
you force the counter again to return to the state
(**Q3 = 0**,
**Q2 = 0**,
**Q1 = 0**,
**Q0 = 0**)...

For this you use an **AND** gate, whose output is connected with the
**RESET** inputs of the flip-flops...

With the circuit shown you realize easily that the counter now produces
only **10 states**...

You find a **mod-10 counter** in many applications
which counts in the **decimal numeral system**.

Now there are also counters that **count down**...