In my project, I have been using the AD7799 and communicating via an AVR microcontroller. I have had no trouble with communication until recently (for months). I am not aware of any changes in the firmware, and none in the hardware. When the trouble first started, I thought I had damaged the board of the AD7799 chip. I have tried both replacing the chip (and some other components) and building a completely new board - same problem.
I can query the chip about all the registers without problems, except the offset register. Is there anything you can think of that would cause problems with this register only?