RTC Drift

[derekkrouze]

Hello,

I'm currently trying to work with the RTC on a Kinetis K61, but I'm noticing quite a bit of inaccuracies and time drift after calibration. I currently have a 32.768KHz (ABRACON ABS07) source attached to the RTC module with battery back up. The ABS07 calls for 12.5pF so I have adjusted the chips capacitors to both 12pF and 14pF on different boards and still have been seeing the same result. Also, there is a 50MHz source driving K61. We are using a PC to look at the 1Hz wave outputted by the chip and adjusting the TCR and CIR to get the output as close to 1Hz as possible. However after calibration I let the board sit overnight and just idle and when I check the time I'm seeing it be as much as 2 seconds slow, which is close to 30ppm. I know temperature causes the crystal to fluctuate but I allowed the boards to come up to operating temperature and stay there and the drift still happens. Also, I don't know if it's coincidence but each bump of the TCR register is about 30ppm which makes me think something isn't being setup in the calibration registers correctly. Any help would be greatly appreciated as I am running out of ideas on how to correct this.

Thanks,

-Derek

[Hui_Ma]

Hi Derek,

The precision/accuracy of the RTC will depend solely on the Crystal precision,it will only depend on the quality of the crystal.

The RTC_CLKOUT also can output 32KHz oscillator clock out, that clock is directly coming from RTC oscillator and customer could measure that clock accuracy.

Customer need to set System Options Register 2 (SIM_SOPT2) [RTCCLKOUTSEL] bit to 0b1 and let RTC CLKOUT output 32KHz clock.

About RTC module external crystal connection, customer can refer K61 reference manual chapter 27.3 External Crystal Connections.

Customer also consider the general layout guideline for hardware design of RTC clock.

More detailed info, please check Kinetis Peripheral Module Quick Reference chapter 4.1.5 Layout guidelines for general routing and placement for more detailed info with below link:

http://cache.freescale.com/files/32bit/doc/quick_ref_guide/KQRUG.pdf

Wish it helps.
best regards
Ma Hui

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