Arria 10 DDR4 SODIMM calibration fails occasionally. How to remove this bug?

I'm assuming the 900 MHz you refer to is the external memory speed. Are you running your user logic at half or quarter rate (and meeting timing)?

Have you considered enabling the EMIF Debug Toolkit feature so you can see the results of calibration and figure out where the issue may be? See this online training for details:

https://www.intel.com/content/www/us/en/programmable/support/training/course/omem1124.html

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