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The lithium battery in many of them is not replaceable either. I made my own circuit for battery backup 20+ years ago, with write protect during power-down and -up. It took a lot of inexpensive components. I suspect that now there are ICs to do the job, but I haven't looked.
Have a look at the DS1210.
Thanks. That's the ticket.
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Will the Microchip Erasable PROM work?
They're all serial, so you can't just connect them directly to the processor bus; but yes, I have used the 24LC256 I²C EEPROM you mention. I had forgotten about the 64-byte page write, which speeds up the writing of large quantities a lot. (I wrote my software years ago and then I forget what's in it and just use it.) I made these half-postage-stamp-sized EEPROM modules with the 8-pin DIPs:
![Image](http://wilsonminesco.com/EEPROMmodule.jpg)
That's a 4-pin socket on the right end, to fit onto a 2x2 header of .025" square posts. The shorting bar on the left is on a 1x2 pin header for write-protect. The LED tells when it's powered down so you can unplug it from the computer board, although this is just the first of many I made and after this first one I put LEDs on the computer to show the status of the I²C power and clock lines.
I intend to make other small I²C modules that can be plugged into the same socket, with an RTC probably being next, just to try it for a future project. My workbench computer keeps time in a software RTC, but a few applications require turning it off. So far I haven't needed the RTC running during those times, but of course if I turn it off and on, the actual time of day won't be accurate. It's mostly used for timing processes though, not so much keeping time of day and calendar.