[6600gt]

Originally Posted by eugenebrooks

Use an optical sensor, such as a banner sensor with reflective tape, that generates one interrupt per rotation of the wheel. Use this interrupt to measure the amount of time that it takes for the wheel to go around with one of the hardware timers, preferably a 16 bit one (timer 1 or timer 3). Use the interrupt from the overflow on the timer to increment a "high word" maintaining a 32 bit result for the time in clock ticks that it takes for the wheel to go around. You can get very high precision for the wheel speed this way with only one interrupt per rotation and any additional interrupts associated with overflow of the timer. The "period" of the wheel is then maintained by the interrupt setup, to be read in the user loop by the PID control for the wheel power, any time it needs to be read.

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There is a race condition in this scheme that you must be careful about.
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Have fun,
Eugene

Dosen't the photo sensor have only a 500us response time?