The M8 chip can be set to anything between 1 and 18hz, so why only 5 10 and 18?

I guess analysing software likes an even number of cycles to process, and if the period isn't a factor of 1000 you can't set a whole number frequency. As is the case for 18hz, this cannot be set exactly, it needs a period of 55.55555555555ms

The factors of 1,000 Answer : 1,2,4,5,8,10, 20, -------------

So disallowing the 20, we can have 6 whole number frequencies, but as we are only interested in 2s and 10s calculations we can also have half number frequencies, as in 12.5hz this is easily set at 80ms period. I think I will give this a try next, as a test of the system.