[avr-chat] How can I individualise specific CPUs?

[Juergen Harms]

I wonder whether there is some approach to provide specific CPUs with 
some program-readable identifier that allows to individually recognise 
each CPU in a way that survives programming operations that start by 
erasing the chip.

To be more clear, here is why I need this: I am having a bus with many 
AT90CAN nodes, each of them driving timed events. The CPU clock of each 
node is used to generate the necessary clock information. I use a 
polynomic correction algorithm to correct hardware-specific clock 
deviation, having a polynome which is specifically defined for each CPU 
(resp. its PCB).

At present, I maintain a list of CPUs and their individual correction 
polynomes and manually introduce them into the object code before a 
device is re-programmed. That is a tedious and error-prone way of doing 
(even if I store the polynome in eeprom and only need to adapt the 
object code when eeprom is re-programmed).

I would like to automatize the manual procedure by some lookup algorithm 
that uses a CPU id as a key. Has anybody come across this problem and 
found a solution? The ideal way would be to have a zone of the eeprom (a 
single byte would be enough) that is protected against 
programming-triggered erasure, and that can be used by the application.

(Note: during normal operation, there is a DCF-driven master timer that 
sends timer messages over the bus, allowing the nodes to synchronize; 
but the system must continue to work even if temporarily the master 
timer goes away - hence the need to adapt each node at least with a 
basic timer-offset correction).