[JimGRobot]

Sorry for the confusion, what I posted earlier only works if you scale your joysticks around zero by subtracting 127 from the joystick reading, and adding 127 back in before writing to the pwm.

So, here is a tweak to my earlier post. Instead of setting the offset to the joystick value, set it to the joystick value minus 127:

p1_y_offset = p1_y - 127;

If the joystick value is 130, and you press the auto trim button, the offset will be 3. When this value is subtracted from 130, you get the null value of 127. When the joystick value is 200, you get 197, and 100 you get 97.

If the offset value is too big, it will limit the range of the joystick value. Suppose you auto trim when the joystick value is 77, the offset is -50. When the raw value of the joystick is 254, the corrected value would only be 204, and if the raw value was less than 50, the corrected value would already be limited at zero.

Play around with this a little. The principle involved is to capture the difference between the joystick's actual value when trimmed, and what you want it to be when trimmed, and apply that difference to future readings.

Jim