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RTC calibration when LSI is the RTC clock

Question asked by Trevor on Nov 23, 2011
Latest reply on Dec 5, 2011 by Trevor
Hi All,
On one of my projects I'm using external watch crystal (32768Hz) as LSE clock to the RTC and all works well. What I now need to do is have the RTC continue to work when the external watch crystal fails. My plan is to detect RTC not moving when CPU is running and then switch to using LSI. However I will need to calibrate the LSI as it has a very high tolerence -- I'm guessing the best way to do this is using the RTC calibration register. SYS clk is from an external 8Mhz crystal so there is a reasonable timing reference. Has anyone got anything they could share/advise for calibrating the RTC when LSI is the RTC clock?