First of all; I'm new to the forum and only know that much about electric engineering etc. so please bear with me!
I've recently got myself an Arduino Pro Mini (3.3V @ 8MHz) and tried to hook it up to a servo just to play around with and explore. However, after many hours spent on trying to locate this problem I finally came to the conclusion that my Arduino board simply can not send PWM-signals properly to a servo whilst being powered through a FTDI connector (http://www.sparkfun.com/products/10009). But the strange part is that if I - after have uploading a sketch - disconnect the FTDI connector and power the Arduino board through it's RAW-input with a voltage in the range of 3-12V instead it actually works perfectly. In the first scenario the servo simply position itself to one of it's greatest limits and keeps drawing current there, while in the latter scenario it does exactly what the sketch tells it to do and works normally. Why is this?
The servos 5V-input is connected to an external power source (for obvious reasons) in both occasions. Depending on if the Arduino board itself is powered through the FTDI connector it uses separate GNDs but otherwise it shares the same GND.
I don't think that I have to submit the actual sketch to this post since I'm more than sure that it is not something software related, but if I'm wrong on this then one could find the very same sketch among the Arduino IDE's example sketches on servos - it's called "Sweep".
Thanks in advance!