Asynchronnous binary counter (ripple counter)

This kind of an asynchronnous binary counter is usually called as "ripple counter". It's compossed of flip-flops and invertors. The invertor is connected between output and input of the same flip-flop and it creates divider by 2. See schematic of four bit ripple counter:

Figure 1 - four bit ripple counter schematic

'clear' signal sets all flip-flops to '1' and all 'bn' to '0'. 'b0' output is changing on rising edge of clock ('clk'). 'b1' output is changing on rising edge of 'b0'. 'b2' output is changing on rising edge of 'b1'. 'b3' output is changing on rising edge of 'b2'.