How to measure frequency of AC Voltage(In range of 0.05-5v) using micro-controller?
Assuming rate of change in amplitude(Vrms or Vpk-Vpk) is very slow compared to the frequencies:


Start a counter which can count the lowest frequency without overflow but with clock frequency larger a bit larger than the highest frequnecy. 
Sample the data using ADC (higher the better but >3000Hz).Adding a low pass filter would be good idea.
When you find the max data point, time-stamp the counter.
Wait to get the min data point, time-stamp. Take the difference, calculate the frequency. 
DONE.


If change of amplitude is also there, then you need to use PLL.