continuous servo only turns in one direction.

I have an arduino pro mini, and am trying to connect a continuous rotation servo motor to the pin 9. but no matter what is outputted to the servo, it just spins clockwise at a constant speed. I have tried 1.25 ms high and 8.75 low, 1.5 ms high and 8.5 low, 1.75 ms high and 8.25 ms low, as well as other combinations to see if my servo was just centered on another frequency or something, or that it needed a different overall period, but still it spings clockwise. I have connected the output to an oscilloscope and it is outputting the correct voltage and duty cycle...I just don't know what to do. I have even tried plugging in another continuous servo motor, and that one still did the same thing.

What am I doing wrong?

Can you show us your code?

Did you mod the servos yourself?
Did you leave the pot in the middle position?

ok so I found the problem, I had the microcontroller powered by one battery and the servo powered by a second seperate battery. but when I created a common ground (connected the negitive sides of both batteries together) it worked perfectly.

thanks though!