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Messages posted by: obelix662000
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Be sure that Tx and Rx are not connected. To get sub microamps sleep current I'm using FETs to disconnect serial port in my project. Otherwise current drain through serial port is in mA range.
I think there is a mistake in the board schematics, in contrast to many others inexpensive RTC MCR7940 does not have built in load capacitors. When datasheet says " 6-15 pf crystal " actually it means that this crystal should be used with corresponding load capacitors. I spent 3 days figuring this out, trying different crystals etc, until I soldered 2 extra caps ( as per document referenced in my previous post). Since then RTC works perfect. Without load capacitors RTC accuracy is not related to crystal accuracy but to self oscillating of some effective RC contour, which gives not only 1 s per hour inaccuracy but also strong dependence on temperature. The board itself is great, this 2 extra caps is not very big deal, but I would recommend to include them in next versions. Thanks for the board, it fits perfectly my needs.
Great board, thanks! Purchased 2 of them, both have problem with RTC, clock gains about 1 sec per hour. It cannot be compensated with calibration register (5 times more than max). Problem fixed with load capacitors connected to crystal in accordance with 20005010F document by Microchip for MCP7940N.

srueger, DS3231 are good, but do not have RAM registers, which are very convenient for many applications. I would prefer DS 3232
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