1. Asynchronous Counters - Asynchronous counters are those whose output is free from the clock signal. Because the flip flops in asynchronous counters are supplied with different clock signals, there may be delay in producing output.

2. Asynchronous 4-bit UP counter - It is capable of counting numbers from 0 to 15. The clock inputs of all flip flops are cascaded and the D input (DATA input) of each flip flop is connected to a state output of the flip flop.

In the asynchronous 4- bit up counter, the flip flops are connected in toggle mode, so when the when the clock input is connected to first flip flop A, then its output after one clock pulse will become 20.

3. Truth Table