Quote Originally Posted by zipphreak
Hi there, I'm looking for some direction in creating an onboard tach for my rc helicopter that reads its input from a hall sensor, mounted on a model engine, that measures engine RPM up to say 25,000 rpm to be safe. Then I divide the read engine rpm by the gear ratio of the helicopter (8.50), to give rotor headspeed in rpm, and output to a 4 digit 7-segment display. Any help would be very much appreciated.
25000 rpm is 417rps(econd). Since the resultant count over a 1 second period is 8.5 x the actual head speed in rps, you can count input pulses for 118mS (1/8.5 second) to get the head speed value, which would result in a number between 0 and 49 revolutions. Multiply this by 60, and you'll get your result in head RPM to output to your display (a surplus Hitachi-44780 LCD display may be much easier to implement).

You could use timer1 configured as a counter with an external clock signal, which should happily continue to count with hardware while you execute a time delay with PAUSE 118. When the time delay is over, you could stop the timer, read and clear it, do the math on the result, and send to your display.

--------------------Picster-------------------