Tim, Hal, Graham, A followup on an old thread...
I have some RV-3028 samples here, both from Digikey and also directly from Micro Crystal. Besides the typical 12 digit BCD date & time format that almost all RTC use, the RV-3028 also has a 4 byte binary seconds counter, which is compatible with unix-style timekeeping.
The BCD format is YYMMDD hhmmss. The chip is spec'd to work until the year 2099. This is typical of digital clocks that use 2-digit years. The year 2100 itself is tricky because it's *not* a leap year. I won't be around in March 2100 to see the off-by-one calendar chaos that follows.
Tim -- you were worried that the RV-3028 will fail in the year 2038. The answer is *no*. The feared 2038 date is related to sign + 31-bit time_t values in unix-like timekeeping. But this RTC uses a 4-byte (32-bit) binary counter, which is implicitly unsigned. So it's fully compatible with both 32-bit and 64-bit unix systems for the rest of the century. Nothing wrong happens to this RTC in 2038.
If you want a 64-bit timestamp just initialize the high 4 bytes to zero once, and then read/reread the RV-3028 counter into the low 4 bytes. This works because the entire upper half of a 64-bit unix timestamp is now and remains zero until 2106 (which is beyond the RTC's 2099 spec anyway). Also you were worried about power; I assume fetching 32-bits is half the power of fetching 64-bits. So maybe give the RV-3028 another chance.
Hal -- your carry detect method would work. But 1) it's not necessary because there is no Y2038K problem with this chip (see above). And 2) Tim says his unique low-power application can't or doesn't want to have to poll the chip or do any math.
Graham -- correct, although Micro Crystal makes some nice TCXO the RV-3028 RTC is not temperature compensated. So if it's extreme accuracy you want instead of extreme low power, this chip isn't for you.
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