PWM pulse resolution

Hi,

I would like to find out what is the minimum resolution of the PWM signal that the AX500 can recognise ? The document say 0.4 %. I would like to know what is this in terms of milliseconds. Also I would like to know if it is a linear relationship between the pulse-width and the rpm.

Thank you and best regards
Edwin
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The timer used to capture the pulses will measure time with 250ns increments. In the end, the measured pulse is turned in a number from -127 to +127. -127 is the 1.0m (after calibration), +127 is the 2.0ms (after calibration)