iMX6 Crystal Oscillator operation at boot

[nick_king]

Hi, I have a system using an iMX6Q (MCIMX6Q6AVT10AD_LB) which shows unexpected problems at boot regarding the 32kHz clock oscillator. The fuses are set to boot from eMMC.

If I allow uboot to auto-run, whatever delays it has built in are tolerant of the 300ms that the external 32kHz crystal takes to start.

If I stop the crystal by using a shorting capacitor, then uboot does not run, and won't recover if I then un-short the crystal.

If I connect a scope probe to the capacitor as well, with no 32kHz seen, the iMX6 boots: noise is shown on the scope so I presume that there are clock edges being injected and detected.

Once the iMX6 is booted, I can stop the oscillator and the processor keeps running OK.

According to the Applications Reference Manual Rev 5 06_2018 section 71.4.1, there should be an internal 32kHz oscillator running when the crystal is not. Has this been implemented in my iMX6 or is there a hardware erratum? Or is there a way of disabling (or enabling!) this clock?

Thanks

[Yuri]

Hello,

    There is a short period during device power-up when the i.MX6 is using the internal ring

oscillator until the crystal oscillator is running. Once the crystal oscillator is running,
the i.MX6 automatically switches from the internal oscillator to the crystal oscillator.

   If RTC_XTALI is tied to GND and RTC_XTALO is left unconnected, the on-chip
oscillator is automatically engaged.

 The following items from i.MX6Q Errata also should be taken in considerations:

ERR007926 ROM: 32 kHz internal oscillator timing inaccuracy may affect SD/MMC, NAND, and
    OneNAND boot [i.MX 6Dual/6Quad Only]

ERR009678 ROM: SD/EMMC/NAND prematurely times out during boot [i.MX
     6Dual/6Quad Only] 

https://www.nxp.com/docs/en/errata/IMX6DQCE.pdf 

Have a great day,

Yuri

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