I have been wanting to do something very similar for my 07 nightster, I bought an arduino and a chip called the elm322 which is supposed to interpret the J1850 codes but I have yet to get it to work, I am more of a programmer than an engineer...
obdpros.com has complete modules and their chips have a few more modes. It is what I used before I went J1850 direct.
1. When you say AVR are you referring to the Arduino or another AVR chip?
2. If it is another AVR I assume this is where your C code goes, or is the C code on the arduino, I thought the Arduino had it's own Processing language?
Arduino, but I do C language directly.
3. Could you post any schematics or a tutorial of how you actually interface with the J1850 data (I don't want to blow up my Arduino)
I post things occasionally on my dev blog - among others:
http://harleyhacking.blogspot.com/2009/ ... -more.html
J1850 connects to ICP1 which is port D8 in Arduino terms through a 5k resistor (4.7k, 5.1k, somewhere around there - it doesn't have to be exact). You need a common ground and can probably power the arduino if it has a good regulator (like the pro does) and you don't do anything which will pull a lot of current on the 3.3v. I use an external regulator because the GPS chip and bluetooth together are pushing the limit.
The page above has the current source code for "j1850.c" which outputs the data stream to the serial port at 57600 baud.
This is probably overload, so feel free to ask follow-up questions. I am assuming the nightster has a similar diagnostic connector (one of the four pin connectors with only 3 connected - accessory, ground, and data).