udev

The role of udev role:

• create a symbolic link to port (e.g., /dev/ttyUSB0) that may change after re-plug
• configure settings about the port, such as badurate, latency

How to create udev rules

1. Execute $ sudo udevadm info -a /dev/ttyUSBX to get information of your device. X is your device's number. An example is following:

$ sudo udevadm info -a /dev/ttyUSB0
Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/3610000.xhci/usb1/1-4/1-4:1.0/ttyUSB0/tty/ttyUSB0':
    KERNEL=="ttyUSB0"
    SUBSYSTEM=="tty"
    DRIVER==""

  looking at parent device '/devices/3610000.xhci/usb1/1-4/1-4:1.0/ttyUSB0':
    KERNELS=="ttyUSB0"
    SUBSYSTEMS=="usb-serial"
    DRIVERS=="ftdi_sio"
    ATTRS{latency_timer}=="1"
    ATTRS{port_number}=="0"

  looking at parent device '/devices/3610000.xhci/usb1/1-4/1-4:1.0':
    KERNELS=="1-4:1.0"
    SUBSYSTEMS=="usb"
    DRIVERS=="ftdi_sio"
    ATTRS{authorized}=="1"
    ATTRS{bAlternateSetting}==" 0"
    ATTRS{bInterfaceClass}=="ff"
    ATTRS{bInterfaceNumber}=="00"
    ATTRS{bInterfaceProtocol}=="ff"
    ATTRS{bInterfaceSubClass}=="ff"
    ATTRS{bNumEndpoints}=="02"
    ATTRS{interface}=="FT232R USB UART"
    ATTRS{supports_autosuspend}=="1"

  looking at parent device '/devices/3610000.xhci/usb1/1-4':
    KERNELS=="1-4"
    SUBSYSTEMS=="usb"
    DRIVERS=="usb"
    ATTRS{authorized}=="1"
    ATTRS{avoid_reset_quirk}=="0"
    ATTRS{bConfigurationValue}=="1"
    ATTRS{bDeviceClass}=="00"
    ATTRS{bDeviceProtocol}=="00"
    ATTRS{bDeviceSubClass}=="00"
    ATTRS{bMaxPacketSize0}=="8"
    ATTRS{bMaxPower}=="90mA"
    ATTRS{bNumConfigurations}=="1"
    ATTRS{bNumInterfaces}==" 1"
    ATTRS{bcdDevice}=="0600"
    ATTRS{bmAttributes}=="a0"
    ATTRS{busnum}=="1"
    ATTRS{configuration}==""
    ATTRS{devnum}=="5"
    ATTRS{devpath}=="4"
    ATTRS{idProduct}=="6001"
    ATTRS{idVendor}=="0403"
    ATTRS{ltm_capable}=="no"
    ATTRS{manufacturer}=="FTDI"
    ATTRS{maxchild}=="0"
    ATTRS{product}=="FT232R USB UART"
    ATTRS{quirks}=="0x0"
    ATTRS{removable}=="unknown"
    ATTRS{serial}=="A907HR6F"
    ATTRS{speed}=="12"
    ATTRS{urbnum}=="24761"
    ATTRS{version}==" 2.00"

  looking at parent device '/devices/3610000.xhci/usb1':
    KERNELS=="usb1"
    SUBSYSTEMS=="usb"
    DRIVERS=="usb"
    ATTRS{authorized}=="1"
    ATTRS{authorized_default}=="1"
    ATTRS{avoid_reset_quirk}=="0"
    ATTRS{bConfigurationValue}=="1"
    ATTRS{bDeviceClass}=="09"
    ATTRS{bDeviceProtocol}=="01"
    ATTRS{bDeviceSubClass}=="00"
    ATTRS{bMaxPacketSize0}=="64"
    ATTRS{bMaxPower}=="0mA"
    ATTRS{bNumConfigurations}=="1"
    ATTRS{bNumInterfaces}==" 1"
    ATTRS{bcdDevice}=="0409"
    ATTRS{bmAttributes}=="e0"
    ATTRS{busnum}=="1"
    ATTRS{configuration}==""
    ATTRS{devnum}=="1"
    ATTRS{devpath}=="0"
    ATTRS{idProduct}=="0002"
    ATTRS{idVendor}=="1d6b"
    ATTRS{interface_authorized_default}=="1"
    ATTRS{ltm_capable}=="no"
    ATTRS{manufacturer}=="Linux 4.9.108-tegra xhci-hcd"
    ATTRS{maxchild}=="4"
    ATTRS{product}=="xHCI Host Controller"
    ATTRS{quirks}=="0x0"
    ATTRS{removable}=="unknown"
    ATTRS{serial}=="3610000.xhci"
    ATTRS{speed}=="480"
    ATTRS{urbnum}=="73"
    ATTRS{version}==" 2.00"

  looking at parent device '/devices/3610000.xhci':
    KERNELS=="3610000.xhci"
    SUBSYSTEMS=="platform"
    DRIVERS=="tegra-xusb"
    ATTRS{driver_override}=="(null)"
    ATTRS{hsic_power}=="0"

2. Create 99-*.rules file under /etc/udev/rules.d/:

• The two-digit number like 99 indicate the priority of this rule file to be loaded. 99 means the highest priority
• Template rule file 99-aerial-robot-dev.rules for serial USB device:

KERNEL=="ttyUSB*",  ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", ATTRS{serial}=="A907HR6F", SYMLINK+="flight_controller"
ACTION=="add", SUBSYSTEM=="usb-serial", DRIVER=="ftdi_sio", ATTR{latency_timer}="1"

• Line1: numbers of idVendor, idProduct and serial should be modified correctly referring to the result of 1.
• Line2: configure the latency: minimum latency of 1

3. Reload udev rules:

$ sudo udevadm control --reload-rules && udevadm trigger

4. Replug your device

you must see the symbolic link under /dev, such as /dev/flight_controller in the above udev rule file.

Tips:

1. Permision of serial port

If you don't have permission to use /dev/ttyUSB*, do $ sudo adduser [username] dialout