i.MX6UL internal SNVS RTC counts too fast

[philippbachmann]

Dear all, 

I want to use the SNVS RTC with backup battery on imx6ul. I was able to enable it in kernel and device tree. From user space I can read it (hwclock -r) and set it to system time (hwclock -w). However, it runs way to fast, around +8 seconds faster per minute. This is more than 137ms per second. According to iMX6UL reference manual there is an RTC calibration register, however the maximum possible calibration value is 16/32768=488us per second, so no way I can compensate it this way. I know that internal clocks are quite inaccurate compared to external crystals, but I don't believe the difference is that big.

Does anyone have an idea what the problem could be? 

Thanks and regards

[philippbachmann]

Hi Igor

Did not expect that the tolerance is that big, thanks a lot!

Best regards

Philipp

[igorpadykov · Accepted solution]

Hi Philipp

according to Table 11. Oscillator and clock recommendations

Hardware Development Guide for the i.MX 6UltraLite Applications Processor

"..on-chip 40 kHz oscillator. The tolerance is ± 50%."

https://www.nxp.com/docs/en/user-guide/IMX6ULHDG.pdf

Best regards
igor
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