I just got my copy of Mastering the I2C bus - an excellent book, the first in Elektor.com's LabWorx series.
The author, Vincent Himpe, really knows his stuff. I've been using the I2C bus since I started with PIC micros back in the last millennium, but I learned something on just about every page of the book.
The book covers the background and history of the I2C bus and explains it from hardware and software perspectives.
There's a useful chapter on FAQs, and the book also covers debugging. The author makes a good selection of commonly used devices, and explains how to use them. The last part of the book describes a series of I2C project boards, (available from an Elektor partner) and covers their purpose, their design and application.
You're encouraged to download code from a dedicated website. I had a problem getting on the site yesterday, but after a fast response from Elektor I now have access to the software and PCB layouts.
We'll be supporting I2C on the board we're making for the Raspberry Pi over at quick2wire.com, and the book is now recommended reading for all the hardware and software developers on the team.
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