"""

    Example code for resetting the USB port that a Teensy microcontroller is

    attached to. There are a lot of situations where a Teensy or Arduino can

    end up in a bad state and need resetting, this code is useful for

"""

import os

import fcntl

import subprocess

# Equivalent of the _IO('U', 20) constant in the linux kernel.

USBDEVFS_RESET = ord('U') << (4*2) | 20

def get_teensy():

    """

        Gets the devfs path to a Teensy microcontroller by scraping the output

        of the lsusb command

        The lsusb command outputs a list of USB devices attached to a computer

        in the format:

            Bus 002 Device 009: ID 16c0:0483 Van Ooijen Technische Informatica Teensyduino Serial

        The devfs path to these devices is:

            /dev/bus/usb/<busnum>/<devnum>

        So for the above device, it would be:

            /dev/bus/usb/002/009

        This function generates that path.

    """

    proc = subprocess.Popen(['lsusb'], stdout=subprocess.PIPE)

    out = proc.communicate()[0]

    lines = out.split('\n')

    for line in lines:

        if 'Teensyduino' in line:

            parts = line.split()

            bus = parts[1]

            dev = parts[3][:3]

            return '/dev/bus/usb/%s/%s' % (bus, dev)

def send_reset(dev_path):

    """

        Sends the USBDEVFS_RESET IOCTL to a USB device.

        dev_path - The devfs path to the USB device (under /dev/bus/usb/)

                   See get_teensy for example of how to obtain this.

    """

    fd = os.open(dev_path, os.O_WRONLY)

    try:

        fcntl.ioctl(fd, USBDEVFS_RESET, 0)

    finally:

        os.close(fd)

def reset_teensy():

    """

        Finds a teensy and reset it.

    """

    send_reset(get_teensy())