RPM resolution

Hi there.

I have this project where i measure RPM in the range of 0-3300. The way it is measured are with a PNP inductive sensor with 2 pulses pr. revolution.

My logic tells me that the resolution is 60 rpm, with a measurement each 500ms.

The way it is calculated:

rpm=30 * 1000 / millis()-millisold * half_revolutions.

half_revolutions is an interrupt on falling edge.

But when starts measuring it also shows values that is not supported. eg. 3187 the resolution says eighter 3180 or 3240?

Can anyone tell me what happens here.

best regards Bastian Gaardahl Christensen

I think the problem has solved iself. The resolution is not 60 RPM. The function measures time between the pulses.