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How to RTC-only mode with AM3358 and TPS65217C PMIC?

Brian Weir
Intellectual 810 points
Other Parts Discussed in Thread: TPS65910, AM3358

I have looked at the EVM's that have RTC battery backup and all use TPS65910 PMIC.  I found the RTC section in the AM335x Schematic Checklist useful.  

The device I am working on will not have power ON/OFF, standby, or low power modes.  The device will be powered ON whenever 5v is applied to the PMIC AC or USB inputs (there will be no battery attached to BAT1 or BAT2).  We want to have a coin cell (or preferably a supercap) supplying RTC battery backup power when the 5v input is not present to maintain the Real Time Clock.

Is there a reference design to support RTC battery backup on the TPS65217C PMIC?  Reading throug the various specs it appears that all that is needed is to supply an indipendent 1.8v to the VDDS_RTC AM3358 input (assuming 1uF attached to CAP_VDD_RTC and RTC_KALDO_ENn pulled low).  Is this correct?  With 1.8v applied to VDDS_RTC, would I expect the RTC LDO to be running and 1.1v on the CAP_VDD_RTC pin?  Also, would I to be able to scope the RTC 32khz crystal for a signal to indicate the RTC is running?      

Is there an accepted method to regulate the VDDS_RTC voltage at 1.8v to reduce load on the battery?

Thanks,

Brian

     

  • 0
    TI__Guru**** 393215 points

    Hi Brian,

    Brian Weir said:
    Reading throug the various specs it appears that all that is needed is to supply an indipendent 1.8v to the VDDS_RTC AM3358 input (assuming 1uF attached to CAP_VDD_RTC and RTC_KALDO_ENn pulled low).  Is this correct?

    Yes, this is correct.

    Brian Weir said:
     With 1.8v applied to VDDS_RTC, would I expect the RTC LDO to be running and 1.1v on the CAP_VDD_RTC pin?  Also, would I to be able to scope the RTC 32khz crystal for a signal to indicate the RTC is running?

    Yes.

    Brian Weir said:
    Is there an accepted method to regulate the VDDS_RTC voltage at 1.8v to reduce load on the battery?

    You should note that the AM335X internal RTC has a rather high power consumption compared to external RTC ICs (http://processors.wiki.ti.com/index.php/AM335x_Power_Consumption_Summary#RTC-Only ). In my opinion the best solution is to use an external RTC with I2C interface.

  • 0 in reply to Biser Gatchev-XID
    TI__Guru 63165 points
    I would like to add a comment related to using a scope to confirm the RTC oscillator is running. This may be possible with some scope probes while others may cause the oscillator to stop oscillating. So you should use a high impedance/low capacitance scope probe when probing the RTC oscillator signals.

    Regards,
    Paul
  • 0 in reply to peaves
    Intellectual 810 points

    Biser,  Paul,

    Thank you for the detailed and exact responses.   I had already looked at the RTC Power Consumption chart but did not realize how much less current a standalone I2C RTC module used, nor how cheap they are.  So, I am taking your advice Biser and have added a RTC module with supercap to power it.

    I will mark this post as answered.

    Regards,

    Brian