STM32H7xx RTC accuracy problems

[JOwen.1]

Have several boards with problems keeping time accurately. We calibrate each board in factory test, but some boards have very large errors in the 1 Hz calibration signal and very jittery frequency readings. To make it more interesting, when there is less activity on the PCA the calibration clock output tends to stabilize at a frequency that can be calibrated.

We have tried changing the 32 KHz crystal and load cap's, but that does not seem to have any effect. Boards that exhibit this problem can loose several minutes of time in 24 hours - not acceptable for a modern clock!

Clues:

1. When the calibration output goes wonky, it typically drops slightly. For example, it might go from a stable reading of 0.999962 Hz to 0.998637 Hz and jump around a good bit.
2. The same board can be good or bad. For example a board on our tester can output a good or bad frequency on a given power cycle. Also, when we load a very simple test program that only turns on the calibration output and not much else, all boards we have tried put out a good calibration signal - even if they were putting out a bad cal signal a minute ago with the original software.

Anyone have any ideas what we can try to get around this problem? Thanks for any advice here!

[JOwen.1]

I have not tried to reset the backup domain registers directly. Our code probably sets them up on boot. Except for drive strength, I was not aware of any other register settings that would affect the basic frequency of the RTC. By the way, I tried different drive settings. Different drive strength settings caused a slight shift in frequency, but over all there was not much difference or improvement to the problems I've been seeing.