Attach any I2C client chip (thermo
sensors, AD converter, displays, relais driver, ...) to your PC via USB ...
quick, easy and cheap! Drivers for Linux, Windows and MacOS available.

The i2c-tiny-usb project is an open
source/open hardware project. The goal of i2c-tiny-usb is to provide a cheap
generic i2c interface to be attached to the usb. It is meant as a replacement
for those simple and cheap printer port to i2c
adapters. A USB solution has several advantages incl. the built-in power supply
and a more future proof interface. Furthermore no cpu
intense bitbanging is required on the host side. This USB solution even
requires less parts than some printer port solutions.

While the i2c-tiny-usb was
developed under and for Linux it also works under Windows and MacOS X. A
windows demo driver and demo application is included to get you started
immediately.

The i2c-tiny-usb project is based
on:

The Linux USB project
and the Linux i2c implementation

The windows driver and test application are based on the libusb-win32

AVR-USB,
a pure software implementation of USB for the AVR platform

USBtiny,
another software usb implementation for the AVR

Hardware

The prototype board including a ds1621
temperature sensor

The final hardware with the same sensor
added

The hardware of the i2c-tiny-usb
interface consists of the Atmel
AVR ATtiny45 CPU, a cheap and easy to obtain microcontroller with 4 KBytes
flash (of which ~2k are used in this application) and 256 Bytes RAM. The
processor is surrounded by few other parts.

The USB interface

The USB interface of the
i2c-tiny-usb interface is based on a pure software implementation and uses two
pins of the AVR (PB0 and PB2). This software implementation supports low speed
USB only which is signalled to the PC by resistor R1.

The I2C interface is implemented
using a bitbanging approach. The hardware supported twi
interface of the attiny45 is bound to hardware pins at the chip that are
required for USB operation and can thus not be used for I2C. The bitbanging I2C
interface being used instead may not be fully I2C compatible and thus not every
I2C client chip may function correctly at this bus. No incompatibilities have
been reported so far. The i2c-tiny-usb provides a software adjustable i2c clock
delay allowing to configure the i2c clock. The default
delay is 10us. Due to additional delays in the i2c bitbanging code this results
in a i2c clock of about 50kHz.

For simplicity reasons all USB
transfers are done via the control endpoint. Since the avr usb library does
only support low speed devices it cannot use bulk transfers which are specified
for high and full speed devices only. Low speed devices support so called
interrupt transfers which are limited to a preset bandwidth while control
transfers can use any free bandwidth (if there's any at all).

The device therefore uses control
transfers for all of its communication. This requires some additional limitation
to prevent multiple driver software (e.g. the kernel driver and the libusb
based test application) to access the device at the same time. Under Linux this
can be achieved by selecting certain access request types. This kind of access
control may not be possible under other operating systems.

Power consumption

The whole device is a so called bus
powered device. This means that the complete device is powered directly from
USB. Therefore the AVR and one or more I2C client chips are powered from the
USB VBUS signal.

The adapter itself draws less than
10mA and reports this to the host via its USB descriptors. The device is able
to power I2C client chips as well. But since these chips vary in power consumption
it is not possible to correctly include their demands into the device
descriptors. It's your responsibility to keep an eye on the total power supply
and especially to make sure that the entire device does not exceed the total
USB limit of 500mA.

It is planned for future firmware
versions to make the reported power consumption software configurable so the
value can easily be adopted to the real power demands
of the entire device.

Kernel driver

The i2c-tiny-usb is meant to be
used with Linux. It comes with a Linux kernel driver that bridges between the
USB and I2C subsystems in the Linux kernel. The driver then attaches to the USB
device and make the i2c bus available to the i2c subsystem. Thus the entire
setup is transparent to client applications like the lm_sensors framework and no special
client chip drivers are required. Instead the drivers already present in the
linux kernel are used with the i2c-tiny-usb as well. With e.g. the ds1621
temperature sensors used in the prototype the output of sensors may e.g. look
like this:

ds1621-i2c-2-48

Adapter: i2c-tiny-usb at bus 003 device 017

temp:     +21.50°C (low  = +15.0°C, high = +10.0°C)  ALARM (HIGH)

Schematics and PCB

The zener diodes in the schematic
are optional. They may be required since the i2c-tiny-usb is directly powered
from the USBs VBUS singnal at 5V. The USB data lines (D+ and D-) are supposed
to be operated at 3.3V only. Some PCs encounter problems at 5V and limiting the
voltage to at most 3.6V may help. My prototype lacks these diodes since my PC
works fine with D+ and D- at 5V.

Resistor R1 is 2k2 instead of 1k5
for the same reason. It is meant to pullup to 3.3V. Since we are pulling up to
5V the higher resistance is required.

Below is the final PCB layout. It
consists of the USB and I2C parts only and does not include a
I2C client chip. Instead it comes with a solder area for easy prototyping. I do
have some of these PCBs left. Just drop me an email if you want to buy one (6
EUR per PCB + 4 EUR shipping). You can easily etch a PCB youself. Since most of
the connections are on the bottom side even a single sided PCB will work.
You'll just have to add the four missing connections using thin wires.

Part placement

Top (click for 600 dpi)

Bottom (click for 600 dpi)

The final PCB

The USB connector space on the PCB
provides two additional holes to allow an USB cable to be directly and firmly
attached to the device without the use of the USB connector. See the image
below for the desired pinout.

Direct cable wiring schema ...

... and in
reality (with pcf8574 client).

Compiling the firmware

The ATtiny45 is quite new and thus
not all parts of the developement chain support this chip in their current
release versions. The latest binutils (linker an assembler) and avrdude (programmer)
support the new chips as well but gcc doesn't. The gcc-4.1.0 has to be patched
to support some AVR cpus incl. the attiny45. Get the latest avr device patches
(patch-newdevices) e.g here.

Uploading the firmware

If you are familiar with Atmel
programming you probably know what you are doing. The Makefile in
i2c-tiny-usb/firmware/Makefile assumes, that you are using
the stk500
for programming the attiny45. Since all 6 user configurable pins of the
attiny45 are required, the so called high voltage serial programming (hvsp)
mode of the attiny45 has to be used. E.g. the stk500,
the AVR-Doper
and the AVR
Dragon are supporting this mode.

Downloads

Version 2007-01-05: i2c_tiny_usb-2007-01-05.zip

firmware version 1.04/2.04 (i2c clock
bug fix)

Test application includes pcf8574
support

Linux kernel driver for tpa81
temperature array sensor

Version 2007-01-03: i2c_tiny_usb-2007-01-03.zip

Bug fixes in usbtiny version

Added PCB layout

Added Windows libusb driver and
ready-to-run test application

Version 2006-12-03: i2c_tiny_usb-2006-12-03.zip

Added support for usbtiny

Firmware version 1.03 (avrusb)/2.03
(usbtiny)

Device supports i2c clock expansion

Linux kernel driver for cmps03
compass module

Updated testapp

Version 2006-11-06: i2c_tiny_usb-2006-11-06.zip

Firmware version 0.01, the first
working version

Kernel driver 0.9

Links

AVR-USB,
software USB for the AVR platform

USBtiny,
another software usb implementation for the AVR

USB user space
library libusb

Win32 port
of libusb

The Linux USB Project

Lm_sensors - Linux
hardware monitoring

User contributions

Henning
Pauls i2c-tiny-usb

Barry
Carters feature extended version

size=1 width="100%" noshade color="#cccccc"
align=center>

Till Harbaum-Impressum

http://www.harbaum.org/till