stepper has 200 steps per rotation

1 rpm 	= 200 steps per minute
	= 3.33 steps per second
	= 300,000 microseconds between steps


500 rpm = 100,000 steps per minute
	= 1666.66 steps per second
	= 600 microseconds between steps


1000rpm = 200,000 step per minute
	= 3333.33 steps per second
	= 300 microseconds between steps


at 8mhz and a 1024 perscaler, this is the slowest speed

9.15rpm = 1831.11 step per minute
	= 30.51   steps per second
	= 32,767 microseconds between steps