The term 'tons' I used only to tell, that it was unneeded.

Also a PLL need still a very clean VCC to work stable. E.g. I faced often problems of jitter also in the case, that the input signal was very stable (derived from a crystal).

And for a lower range of 100RPM you need really a very slow reacting PLL.
Means, you would see a delay of many seconds from changing the speed until the new value was displayed.


In opposition the measuring of the revolution time give always at least after 60/100RPM = 0.6seconds the actual value.

Also to get the right feeling you should never display a new value shorter than after 200..500msec.
So you have all time of the world to do some calculations.

Also no remarkable amount of code space was needed for the calculations.
E.g. I programmed my frequency meter in C on using floating point and nevertheless it fits easy into the 2kB of the AT89C2051.

And if you implemented (or copied) the needed division routine you have also an easy way to convert the binary result into decimal numbers (successive division by 10).


So I see only advantes on avoiding the PLL, but no disadvantages.


Peter