I already had everything on a breadboard. I was working with clock and scheduler code. I was looking at time drift and such. When I needed a frequency source I used the DS1307. It gave me a chance to check that portion of the library as well as gave me something to measure.
In another thread, I was wondering if you could count the pulses from a DS1307 while the processor was asleep. I tried every way I could think of to do just that. It seems that the only counter that will work when asleep is counter 2 and it does not have an external source.
I could get counter 1 to count pulses but only when awake. But back to this thread, the RTC is used as a frequency source and not a time base. The 16MHz clock is used as the time base.
As far as working to my satisfaction, it seems to. I haven't need to use a frequency counter but I was inspired by your writings to try and duplicate your success. I was seeing some strange reading from your code every now and then and I think it was happening when there was an overflow interrupt while reading the overflow counter. In your code, at the beginning of the interrupt handler, the current counter value is saved to local variable. Several instructions later the overflow counter is shifted 16 bits and the counter is added to it. This gives the number of 65ns pulsed in the period. The problem is that in that time, if there is an overflow, the count is off by about 64K. This gives strange results.
Granted, it doesn't happen often, but I did see it a time or two. I loaded your code and fed the 32KHz signal into the Nano. There was the occasional burp. I assumed it was due to an overflow while doing the calculation. There was also the problem of the overflow counter wrapping while sampling. Again, very rare. I have the same problem in my code when the micro() counter overflows.
Thank you for providing so much educational material.