headslave

Firmware for I2C Servo Controller
Works with ATtiny26 and ATtiny2313

Device Description

• The servo controller is able to drive up to 10 servos on an I2C bus
• It produces PWM control signals whose pulse width is adjustable through the I2C interface
• The power supply and interface to the microcontroller is operated at 3.3V to 5.5V
• The servo motors can be operated at a higher voltage, usually 6V
• The firmware is available for 2 versions:
  • The ATtiny26
  • The ATtiny2313
• Each controller is clocked at 4MHz (internal)
• The LED blinks once when power is first applied.
• Thereafter the LED flashes briefly when the chip is accessed via I2C with the correct address

Protocol

1. Send START condition
2. Send address and R/W direction bit. The device answers with ACK.
3. Send 1-10 data bytes. The device answers each byte with ACK.
4. Send STOP condition

Address format

By default, the device address is 0b0001111.

Connecting the address pins to ground can set the corresponding address bit low:

ATtiny2313

Pin 13 = a0
Pin 14 = a1
Pin 15 = a2
Pin 16 = a3

ATtiny26

Pin 4 = a0
Pin 7 = a1
(a2 and a3 are always high)

Unconnected address inputs are set to high.

Data format

• The first data byte controls servo0, the second byte controls servo 1, and so on
• Normal values are in the range 30-160 (about 95% of the average values are in this range)
• The special value 0 switches the PWM signal off.
  • Most data servos then switch to standby - they no longer take countermeasures if external forces act on it

Circuit

Check out the wiki for more information about the circuit.

License

This firmware is released under the GPL license.
See LICENSE.