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AM3359 RTC Debug

Rick Brower
Prodigy 130 points

HI all

I'm working with a customized variant of the BBB, using AM3359BZCZ processor.....

The PMIC is the same as the BBB (TPS65217C).  The RTC circuitry is connected per pg 871 of Doc# SPRUH73J, and VDDS_RTC is powered by external lithium coin cell via step down linear regulator when main power to the board is removed.  I've confirmed that 1.8V is always present on pin D7 (VDDS_RTC), but the RTC keeps resetting it's time to zero when I pull power and reboot.  I'm checking the time by reading RTC_DATE and RTC_TIME from the command line.

So I took an oscilloscope to the 32khz oscillator and discovered that Pin A6 (OSC1_IN) ceases immediately to oscillate when Pin E6 et.al.(VDDS) drops out and simply remains high.  If I subsequently remove the lithium coin cell, of course then it goes low.  On a possibly related note, once main power is reapplied, Pin A6 (OSC1_IN) is delayed by >1 sec before oscillations resumed.  See attached oscilloscope screenshot for reference.

Keep in mind that I am simply pulling the main power, rather than ramping the supplies down in reverse order.  I'm not sure if that is part of the problem; I don't think it is but I could be wrong.  Also, as you can see VDDS is the first supply to turn on, but that is a 1.8V rail and since its not the RTC rail I think it should be booting up later per the sequencing diagram on pg 99 of Doc# SPRS717F.  That is how the BBB is setup though, which might be a contributing factor as well?

Any thoughts?  Thanks.

  • 0
    TI__Guru 63055 points

    What is happening to the RTC_PWRONRSTn terminal?  When using the RTC, it is only reset the first time power is applied to the device and never reset again.

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 130 points

    Ah! that's probably the cause...

    RTC_PWRONRSTn is connected to output from PMIC (LDO_PGOOD) which is triggered by the VDDS rail...so when the rail goes down RTC_PWRONRSTn goes down, even though power is still applied to VDDS_RTC via the battery.  I will verify tomorrow.  Thanks!