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Operating AM3352 with RTC disabled

Elvis Mota1
Prodigy 80 points
Other Parts Discussed in Thread: AM3352, TPS65910

Hi,

I'm designing a board using AM3352 where I will disable the RTC by pulling down RTC_XTALIN with a 10k resistor and connecting  VSS_RTC to ground. Should RTC_PWRONRSTn be pulled down as well.

In order to not have to source CAP_VDD_RTC with an external 1.1V supply, I'll pull down RTC_KALDO_ENn and also provide 1.8V to VDDS_RTC.

The PMIC_POWER_EN output cannot be used because the RTC was disabled by not providing an input clock, even though the internal RTC_LDO will be on. Is this correct?

The power supply sequencing on page 95 of datasheet SPRS717D –OCTOBER 2011–REVISED MAY 2012 does not indicate any detailed information about how long the PWRONRSTn input should be asserted or if there is a minimum of cycles CLK_M_OSC must go before releasing the PWRONRSTn. I'm using a different power supply solution than the TPS65910.

Regards,

Elvis 

  • 0
    TI__Guru**** 393215 points
    Hi Elvis,
     
    RTC_PWRONRSTn should be driven together with PORZ, because the RTC module still needs to be reset even if not used. Note, however, that RTC_PWRONRSTn is on the VDDS_RTC domain, while PWRONRSTn is on the VDDSHV6 domain. If VDDSHV6 is 3.3V you will need to buffer RTC_PWRONRSTn with a 3.3V input-tolerant buffer, supplied from VDDS_RTC.
     
    Supply is correct, but you will need to add an 1uF capacitor to CAP_VDD_RTC.
     
    You are correct about the PMIC_POWER_EN signal, it can't be used in your case.
     
    About PWRONRSTn timing, Datasheet Table 4-3 and Figure 4-9 give a typical oscillator startup time, This can be used as a guideline for reset release timing, but individual board factors can also influence this timing, so you should allow for these.
     
    Best Regards
    Biser
  • 0 in reply to Biser Gatchev-XID
    TI__Guru 62855 points

    Please reference the latest AM335x Data Sheet, AM335x Silicon Errata, and AM335x Schematic Checklist for information related to RTC usage and requirements.

    For example, previous discussions in this post states RTC_PWRONRSTn terminal should always be driven together with the PWRONRSTn terminal even when the RTC is not used.  Later it was determined the RTC_PWRONRSTn terminal could be permanently connected to VSS when the RTC is not used.  This information was updated in the AM335x Schematic Checklist, bit not in this post.

    Regards,
    Paul