29 Aug 2003
(updated 29 Aug 2003 at 17:44 UTC) »
Fun with I2C
Work on the Open
Project the last few days has focused on getting the IIC
bus up and running with the VIA Nehemiah M10000 EPIA M
Mini-ITX motherboard which is at the heart of my prototype
The I2C bus is the backbone to which many of the
microcontroller-based driver and sensor boards connect. This
arrangement keeps the wire runs around the robot simple
(basically just power and IIC), and keeps most of the
mainboard ports free for other functions.
After a few trials and tribulations, I finally got the
whole thing working really well, so I thought it would be
worth chronicling some of the notable things I found for the
benefit of others who may also be attempting to access I2C
peripherals on their PC-based robot (running GNU/Linux).
I found this
web page a useful source of information. It describes
how to access I2C devices using the /dev interface
from the PC's standpoint. If you're using the same VIA
mainboard as me, then you'll need to
load the drivers i2c-viapro and i2c-dev. For
example, you could add the following lines to a startup
script such as /etc/rc.d/rc.local
To check if the devices are loaded, typing
cat /proc/bus/i2c should list the I2C buses on
your system. On my system, there's one entry, called
i2c-0. This device is accessed from your code using
the device name /dev/i2c-0, as described in the web
page I linked to earlier.
I found that on Red Hat Linux 9, everything worked "out
of the box". There were no patches to apply or drivers to
One thing worth noting about the hardware implementation
of the I2C interface on the VIA motherboard, is that it uses
3.3V logic rather than the usual 5V logic. This is no big
deal because all I2C signal lines are open collector anyway.
It just means that any pull-up resistors you use be should
tied to the 3.3V rail
that's conveniently provided on the mainboard I2C connector,
rather than a 5V rail. This last point may actually be moot,
because based on the results of my experimentation, it
appears that the host mainboard has its own on-board pull-up
resistors, and the interface seemed to work perfectly well
without pull-up resistors at the peripheral end.
I'm using PICmicro devices on the I2C bus, configured as
slaves (obviously, the mainboard is the master), and I found
a couple of very useful documents to help me with this.
One is Microchip's Application Note AN734, "Using the
PICmicro SSP for Slave I2C Communication", and the other is
the official SMBus Specification, version 2.0. The second of
these documents describes how the SMBus protocol can be
implemented using I2C, allowing the higher level
i2c_smbus_ functions to be used at the PC end (highly
recommended). I've provided links to PDF versions of both of
these documents at the bottom of the downloads page of the
Open Automaton Project web site.