Hi,

The counting method will mainly depend on the input pulses frequency ...

COUNT need lots of ... counts and if pulses frequency is low ...

Response time is very slow ...

One must try to make a PERIOD measurement, then. and better calculate a mean value over some cycles ...

the thing I like to do is have a big counter and fill it with sum of period samples ... so the sampling time remains quasi constant.

few long samples if engine runs slow, many short ones if engine runs fast ...

I also use a switchable 1:10 divider ( a 4017 ! ) upon the RPM range ...

This way, I had realised a model plane RPM meter ranging from 43 to 32,000 rpm ... with more than .1% accuracy and an acceptable response time ...

The interrupt way is trickier, because of false triggering to avoid ... so, a very well designed input stage has to be used ...

But it's the fastest response ( for shift Lights, i.e. when max. engine accel. is looked for ...)

Alain