Bus Pirate servo driver documentation
Bus Pirate servo mode. Taken from the PDF file by Randy.
Servo mode is used to test RC hobby servos using the AUX pin on the Bus Pirate. The PWM generation is very similar to the ‘g’ signal generate mode.. Servo mode is launched from the user menu with a capital ‘S’ and has a range of values from 0 to 180 which represent the servo arm angle of 0 to 180 degrees. 0 degrees produces a pulse width of 0.990 mS and positions the servo arm fully counter clockwise. 90 degrees produces a pulse of 1.500 mS and positions the servo arm to the center position. 180 degrees produces a pulse width of 2.010 mS and positions the servo arm fully clockwise. Most servos have a specified range of 1 to 2 mS for full travel.
The AUX pin should be connected to the control signal pin of the servo . The Servo will also need to be powered from a suitable power supply or battery. Even small servos can require several hundred milliamps of current and the Bus Pirate power supply is not capable of supplying enough current.
Most remote control models use 4.8 to 6V batteries or a battery eliminator circuit (BEC) to power the servos and radio equipment. You will also need to connect the GND pin of the Bus Pirate to the common lead of the servo power supply. I have tested small servos powered from USB with good success.
The period of the servo signal is 20 mS as shown in this oscilloscope shot.
Here are some photos of my lashup showing the servo positions at 0, 90, and 180 degrees, and the generated pulse widths.