I lately came across the need to use a dual edge counter, by this I mean a counter which is counting both on the rising and on the falling edge of the clock.

The limitation is that one has to use only normal single edge sensitive flops, the kind you find in each library.

There are several ways to do this, some easier than others. I would like to show you a specific design which is based on the dual edge flop I described in a previous post. This design is just used here to illustrate a point, I do not recommend you use it – there are far better ways. Please refer to the end of the post for more on that.

The figure below depicts the counter:

The counter is made of 2 n-bit arrays of flops. The one operates on the rising edge, the other on the falling edge. The “+1” logic is calculated from the final XOR output, which is the real output of the counter! The value in each of the n-bit arrays does **not** represent the true counting value, but is **used** to calculate the final counter value. Do not make the mistake and use the value directly from either set of flops.

This leads to a small puzzle – given the conditions above, can this counter be done with less flops?