USBpwn source code for AVR USB device
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README.md

USBpwn AVR device

The purpose of this device is to receive any (text or binary) file using the LEDs on the keyboard (Num, Caps and Scroll lock). It uses the V-USB library to act as a USB HID keyboard, and is the modified version of one of the projects using it (c64keys, which in turn used HIDkeys).

Storing data

First, it sends a text-based script from its Flash memory as keystrokes (SD2KEYS mode), which can be used to act as a host helper in sending data in the next step. After the last keystoke is sent, the device turns into KEYS2EEPROM mode, and it monitors the LED updates mentioned above. The protocol used is the following:

  • when the Scroll lock is turned on, the Num and Caps lock state is sampled
  • the Num and Caps lock state is the last two bits sent (in this order)
  • the device sends an Enter/Return keystroke

In other words, Scroll lock is used as a "clock signal", and at raising edge, the two other LEDs can be sampled for data. The newline (that can be achieved by the Enter/Return key) is the acknowledgement signal, making the protocol fully synchronous. For example, sending the bits 1101 can be done in the following way:

            __________________________________
   NUM ____/
                               _______________
  CAPS _______________________/
                ______            ______
SCROLL ________/      \__________/      \_____

                  01                11

Bits are sent from LSB to MSB, n bytes are sent from 0 to n-1, stored at the nth position in the EEPROM. The MCU I used is the ATmega328, which has 1 kbyte of it, which limits the size of the largest file that can be received.

Retrieving data

If the 5th bit of PORT B (digital 13 on Arduino) is pulled to ground upon boot, the device starts up in EEPROM2KEYS mode, in which it sends the contents of the EEPROM bytes in hexadecimal encoding. It doesn't know which cells were used last time, so it sends all bytes in ascending order in an endless loop. One easy way of decoding it is using the unhexlify method of the binascii Python module.

Dependencies

  • GNU AVR C library
  • Objective Development V-USB library
  • Python with serial module (for Arduino upload only)

Compiling

Install the dependencies and run make hex. The main.hex file contains the compiled image, which can be uploaded to Arduinos using upload.sh. If your serial port is not at /dev/ttyUSB0, you'll need to change it there.

License

The whole project is under OBDEV license.