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Servo Board v4 Firmware

The Servo Board can be used to control up to 12 RC servos. Many devices are available that can be controlled as servos, such as RC motor speed controllers, and these can also be used with the board.

These are split into 2 groups:

  • 0-7 have a fixed 5.5V output, supplied from the onboard regulator
  • 8-11 have their output voltage supplied from the AUX header and can be provided up to 12 volts

Instructions

Using a posix system, you require make, the arm-none-eabi toolchain and git. Before attempting to build anything initialise all the submodules.

$ git submodule update --init --recursive

To build the main binary, run:

$ make

The binary will then be at src/main.bin. This will also build the library libopencm3 the first time you run it.

This can be flashed to an attached servo board that has a bootloader using:

$ make -C src dfu

To use the dfu command you need to install dfu-utils. This is a cross-platform utility.

To build the bootloader, run:

$ make -C bootloader

The bootloader binary will then be at bootloader/usb_dfu.bin

LEDs

There are 14 LEDs accessible from the firmware, 12 associated with servo outputs and 2 general status LEDs. The 12 associated with servo outputs are lit when the corresponding output is enabled. The blue status LED is lit once the MCU has initialised. The red status LED is lit when there is an I2C communication error, likely due to missing 12V power.

USB Interface

The Vendor ID is 1bda (University of Southampton) and the product ID is 0011.

The Servo Board is controlled over USB serial, each command is its own line.

Since the serial port is virtual, the baudrate is unused and can be set to any value.

Serial Commands

Action Description Command Parameter Description Return Return Parameters
Identify Get the board type and version *IDN? - Student Robotics:SBv4B:<asset tag>:<software version> <asset tag>
<software version>
Status Get board status *STATUS? - <Watchdog fail>:<pgood> <Watchdog fail> - watchdog timeout, int, 0-1
<pgood> - power good, int, 0-1
Reset Reset board to safe startup state
- Disable all servos
- Reset the lights
*RESET - ACK -
Disable output Disable a servo output,set to 0 pulse width SERVO:<n>:DISABLE <n> servo number, int, 0-11 ACK -
Set position Set the position of a servo SERVO:<n>:SET:<value> <n> servo number, int, 0-11
<value> servo duty time, us, 500-4000
ACK -
Get position Get the current position setting for a servo SERVO:<n>:GET? <n> servo number, int, 0-11 <value> <value> servo duty time, us, 500-4000
Read servo current Get total 5V current draw SERVO:I? - <current> <current> - current, int, measured in mA
Read servo voltage Get 5V SMPS output voltage SERVO:V? - <voltage> <voltage> - voltage, int, measured in mV
Debug servo values Print the internal values for servo pulses *DBG:SERVO:<phase> <phase> - Servo output phase number, int, 0-3 <phase data> <phase data> - Raw servo phase values

udev Rule

If you are connecting the Servo Board to a Linux computer with udev, the following rule can be added in order to access the Servo Board interface without root privileges:

SUBSYSTEM=="usb", ATTRS{idVendor}=="1bda", ATTRS{idProduct}=="0011", GROUP="plugdev", MODE="0666"

It should be noted that plugdev can be changed to any Unix group of your preference.

This should only be necessary when trying to access the bootloader.

Designs

You can access the schematics and source code of the hardware in the following places.

Packages

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