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RTC behaviour during power loss

reto hermann
Intellectual 720 points
Other Parts Discussed in Thread: TPS65910

Dear support team,
I have a question regarding the behaviour of the RTC with silicon rev. 2.0 of the AM335x with the following szenario:

(1) All power supplies off
(2) Cold reset of the complete system
(3) The software initializes the RTC and the system is in active mode
(4) Power loss of the main power supply, the RTC domain remains powered with a backup battery
(5) POR of the main supply, system initializes and remains in active mode 

Questions:
- What happens with the system time during (4) and (5)? Is the time information lost and/or the RTC oscillator stopped during power fail or POR of the main supply?
- Is there an increased supply current of the VDDS_RTC supply because the AM335x is in "active" instead of  "RTC only" mode?

Best regards
Reto

  • 0
    TI__Guru**** 393215 points
    Hi Reto,
     
    If you are referring to Errata 1.0.5 in your scenario, this has been fixed in silicon Rev.2. Please refer to the new Errata document on the AM335X product page.
     
    As to VDDS_RTC currents, there are no figures available yet. If you are referring to the previous post (VDDS_RTC supplied from coin cell via external LDO), I would suggest that you supply the LDO input through a double Schottky diode (e.g. BAT54C) - anode 1 to coin cell, anode 2 to 3.3V power supply, cathode to LDO input. This will help preserve the coin cell while the device is powered on.
  • 0 in reply to Biser Gatchev-XID
    Intellectual 720 points

    Hi Biser,
    thank you for your reply. I'm not referring to Errata 1.0.5 in my scenario. I have seen that the gating of the clock is fixed now in silicon rev. 2.0. My concern is, that the RTC could be stopped or the system time could be lost because the AM335x is not entering RTC only mode after a power loss of the main supply.  After reading the TRM spruh73g.pdf  / section 8.1.4.3.6 RTC-Only/ Figure 8-5 I have had the impression that the RTC-only mode is entered by software. During a power loss the software has no chance to enter this mode and I was wondering what the RTC is during in this case. 

    Best regards
    Reto

  • 0 in reply to reto hermann
    TI__Guru**** 393215 points
    Understood. I have forwarded this, as it's an undocumented case. You should receive a reply on this thread.
  • 0 in reply to Biser Gatchev-XID
    TI__Guru 62945 points

    RTC only mode assumes the product always provides a valid power source to the RTC while operating in this mode.

    When power is first applied, the RTC will automatically turn-on the other AM335x power supplies via the PMIC_POWER_EN output going high.  After the AM335x has booted the application software, the application can configure the RTC to generate periodic interrupts as well as turn-on and turn-off events. Once the RTC is initialized, it will continue to run as long as it has a valid power source, a 32.768 KHz clock source, and RTC_PWRONRSTn is not asserted.

    To enter RTC only mode, the application software will schedule a turn-off event in the RTC which causes the PMIC_POWER_EN output to go low and this turns off all AM335x power supplies except the RTC power supply.

    As mentioned above, this mode of operation requires the RTC to always have power applied.  If the product has an unexpected power failure that affects the RTC power source, the RTC would not be able to retain its configuration. In this case, once power is restored the product would restart and go through the same start up process described above.   If the product has an unexpected power failure that does not affect the RTC power source and the RTC was previously scheduled with a turn-on event that occurs after power is restored, the RTC would retain its configuration and turn-on at the previously scheduled turn-on time.

    Does this answer your question?

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 230 points

    Hello TI support team,

    Looking in TI reference designs EVM,IDK and ICE, don't see a mechanism  supporting  ext_wakeup event for waken up the AM335x from RTC-only mode

    I'm using the TPS65910 and have an on/off switch, can you reccomend how to generate  ext_wake event  uppon pressing the on-off switch (which is also connected to PMIC pwron input )?

    Currently I'm using PMIC  int1 output pin to waken-up the CPU from deep sleep modes by connecting PMIC INT1 output pin to one of CPU BANK0 gpio's , I'm looking also for a way to waken up the CPU from RTC-only mode by generating an ext_wakeup  triggered by pressing the on/off switch 

    Yoni Wortzman

  • 0 in reply to yoniw
    TI__Guru**** 393215 points
    Hi Yoni,
     
    Check section 8.1.4.3.6 in the AM335X Technical Reference Manual.
     
    Please, in the future post your questions on new posts. This makes it easier for us to track them and also doesn't deviate the topic of other people's threads.