Cheapi2c
by andy@warmcat.com 

Some wire makes a very useful 100% capture
i2c protocol monitor from your Linux PC
     
 Note   This project has been built and tested.  It is easy and suitable for beginners.
 
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Cheapi2c

Notes -

1) I am indebted to Milosch Meriac (www.meriac.de) for the code allowing the usermode app to fully take over the CPU for the duration.  You can find this code in CMiloschUserRealtime.h in the milk sources.  Its obviously fantasically useful for some kinds of hacks, and very smart and selfcontained code too.

2) This code only runs on Linux at the moment.

Scope 

Cheapi2c uses your PC CPU and printer port to perform realtime snooping of a standard 100kHz I2C bus with 100% capture.

Overview

Cheapi2c intensively scans activity on two printer port input lines and interprets changes on these lines according to the I2C protocol, all in realtime.  When capture is completed, the timestamped activity is dumped to stdio.

Principles

  • No powersupply required
  • PC CPU used to perform capture - to ensure no dropouts, CPU is completely taken over for duration of capture
  • Diodes do not need to be fitted (they are there for later software enhancements allowing I2C writing from the PC)
  • Uses the lmilk app like all the other projects on the site
  • Cheap, unbuffered design means it will not work with some poorly designed printer ports, but will work with 99% of the ports out there.

Implementation



You need a 25-pin male D-connector, preferably with solder bucket termination (these are little cups that make it easy to solder wires to the connector).  I only had a rightangle one lying around the office, of course that worked fine.  You can find these lying around on scrap ISA printer port expansion cards.  Here is a little reminder of which pins are which on a 25-pin male D-connector:

This is the schematic, note that the diodes are not required, but if used they must be BAT42 or equivalent SCHOTTKY diodes.  The diodes are required only to allow the PC to write on to the I2C bus, the software for this function is not written yet.  The pin numbers on the right refer to the XBox LPC connector.
 
Here are the pin numbers of the XBox LPC connector in full.


Here is a closeup of the wiring seen from the front:



... and from the rear:


Software

You just use lmilk like this

lmilk -2

The -2 switch puts lmilk in i2c mode.

Milksop GPL Reflasher - 0.11 - (c)2002 andy@warmcat.com

CheapI2C on printer port at 0x378
Starting realtime...
Entering capture mode, your PC will appear frozen - press any key to end
      0uS: 45 W  -nak-
    312uS: 45 W BA 3F
 10.067mS: 45 W BA 3F
  2.030mS: 45 W 6C 46
  2.017mS: 45 W B8 00
  2.029mS: 45 W CE 19
.....
492.132mS: 10 W 11
    289uS: 10 r 02
Done

Note that after the Entering capture mode... line, your PC will seem to be completely frozen, even the mouse pointer will not move.  Activate the device with the I2C sequence you want to monitor, when it is done press any key on the keyboard to get the dump.  The dump can easily be redirected to a file using something like lmilk -2 >temp.txt

The first field in the dump is the time elapsed since the start of the previous line to the start of this line.  Next is the I2C address the access was associated with (note this is a 7-bit address), then r or W showing if it was a read or write.  Then comes the bytes of data that were read or written.