Hi Vivek,

using a PLL add not only tons of unneeded circuits, its still not easy to design and also very sensitive against any noise, e.g. on the VCC.
Furthermore a PLL react damned slow on changing of the input frequency and your measurement was delayed many times.

So you should avoid PLL whenever you can !!!

And you can in this case.
Your approach was right, simple measure the period time of one or more periods.
Then with f = 1/t you can calculate the frequency respective RPM.
And any µC love it to make the calculations for you.

Search on the web or look at:

http://www.specs.de/users/dan.../index.htm

to found the needed arithmetics for every accuracy, which you want. I suppose 32 bit should be sufficient in your case.


Peter