Hello!
My Idea for auto ranging in the frequency counter is that, you define two or more possible routines in your program according to the various scales that you want e.g. *10, *100 and so on. The idea is that you monitor the incoming signal, with reference to an internal counter.If your counter overflows during, say, one cycle of the incoming signal, you reduce the frequency of your internal counter. That ways you finally get a certain count value, based on one cycle of incoming signal. Since you know the freq of your internal counter, using its count, you can calculate the frequency of the incoming signal.