Perhaps a unrelated interrupt happening during low-level RTC accesses, resulting in stretched/incorrect transfer timing?
I have no idea, the RTC appears to have been reset (i.e the registers have gone back to "zero"), as far as I can tell this can only happen if power is removed from the RTC. The RTC battery backup is directly connected to the RTC, so even if the main power goes, the battery kicks in. On powerfail you cannot write to the registers of the RTC.
Infact, the only time we write to the RTC is when the device receives instructions to do it.
i2c (afaik) should be immune to timing, you can go as slow or fast (as long as the device can keep up) as you want. It's not even like a single register is getting corrupted, the whole thing just "resets".
It's very odd. The battery on it should be good for 20 odd years!
Thing is that these particular pieces of equipment are sat on poles by the side of the road, so it's not a convenient place to debug!
Adrian
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