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RTC_PWRONRSTn connection when used in RTC functionality mode without RTC-only mode

Dong Hui
Prodigy 220 points
Other Parts Discussed in Thread: AM3352, TPS65910

In our design, we need the RTC function in AM3352 but no RTC-only mode. We also use TPS65910A3A1RSL as the power management IC. The connection between the two is pretty much the same TI starter kit design with the exception of CAP_VDD_RTC and RTC_KALDO_nEN connections (CAP_VDD_RTC is connected to VDD_CORE and RTC_KALDO_nEN is connected to VRTC.

However, in the datasheet, it is also mentioned that when using RTC functionality without RTC-only mode, RTC_PWRONRSTn can be connected to the PWRONRSTn. In our case, PWRONRSTn is connected to the nRESPWRON pin of the PMIC, in which case, whenever the PMIC is turning on, the RTC will be reset back to default losing the time. Is my understanding correctly?

We need to keep the time even when PMIC is off. VDDS_RTC is powered by a separate 1.8V LDO which is always on even when PMIC is off. Instead of connecting RTC_PWRONRSTn to PWRONRSTn, can we connect it to VDDS_RTC reset circuit (which is at least 1ms delay after the VDDS_RTC rail turns on?

  • 0
    TI__Guru**** 393215 points

    Dong Hui said:
    We need to keep the time even when PMIC is off.

    This is exactly RTC-only mode. Please follow the guidelines for RTC-only mode given here: http://processors.wiki.ti.com/index.php/AM335x_Schematic_Checklist#RTC 

    In your case you will need to connect either:

    1. CAP_VDD_RTC to external separate always-on 1.1V LDO (not to VDD_CORE) and RTC_KALDO_nEN to VRTC, or

    2. CAP_VDD_RTC to 1 uF decoupling capacitor to GND and RTC_KALDO_nEN to GND.

  • 0 in reply to Biser Gatchev-XID
    Intellectual 560 points

    Hi Biser,

    I am the colleague of Dong hui and thank you for the reply. we still have something unclear , let us have a further discussion.

    our hardware design :

    VDDS_RTC : connected to a 1.8V dedicated power from battery. it will be always on as long as the battery isn't flat out.

    nRTC_KALDO_EN : connect to VDDS_RTC. that means the internal LDO is disabled.

    CAP_VDD_RTC: connect to VDD_CORE

    our question is as below.

    1. does the RTC features function well? ( the RTC is supposed to keep time running );

  • 0 in reply to Jason Z
    TI__Guru**** 393215 points
    When you place the PMIC in sleep mode it will turn off all voltages to the processor, except VDDS_RTC. In your case VDD_CORE will be turned off and the internal 1.1V voltage necessary for RTC operation will be down. The RTC will not operate without this 1.1V supply.
  • 0 in reply to Biser Gatchev-XID
    Prodigy 220 points

    I'd like to summarize our discussion here.

    First of all, according to the AM335x datasheet and schematic checklist, the RTC functionality can be configured to one of the three scenarios.

    (1) RTC-only mode

    (2) RTC timer functionality but no RTC-only mode

    (3) RTC feature disabled

    For (2), it is described in the checklist that "if you will be using the RTC feature but don't need RTC-only mode. This user case allows you to use the real-time clocking features (e.g. keeping time), but you don't need to support the low power RTC-only mode".

    Further to Biser's above explanation, my understanding for (2) is, the real-time clocking features (e.g. keeping time) only works when AM335x is being powered up. If the hardware configuration is set to (2) AND the AM335x processor is NOT being powered up (i.e. only VDDS_RTC pin is powered on by an always-on 1.8V power supply and all other rails of AM335x are off), then time-keeping will NOT work.

    Is my understanding correct?

    Therefore, the only way to have time-keeping works when AM335x is off, is to have the hardware configuration set to (1) - RTC-only mode.

  • 0 in reply to Dong Hui
    TI__Guru**** 393215 points
    Yes, your understanding is correct.
  • 0 in reply to Biser Gatchev-XID
    Prodigy 220 points
    Thanks for the confirmation!

    I have another question regarding the power consumption in RTC-only mode. On the AM335x starter kit, I have measured the current on the VRTC rail (which is powered by a LDO TPS71718) while the AM335x is in RTC-only mode and PMIC (TPS65910) is OFF.

    The current consumption is about 0.35mA, which is a little bit high for our product.

    I have been advised that the high current consumption is due to the internal LDO. To reduce the overall current consumption, we can disable the internal LDO by pulling RTC_KALDO_ENn to VRTC and then have an external 1.1V LDO (low quiescent current) connected to the CAP_VDD_RTC pin. However, I was not advised if it is done this way, what the estimated current consumption would be on both the 1.8V RTC and 1.1V LDO? Less than 100uA?

    Is this a feasible plan to reduce the current consumption in RTC-only mode?
  • 0 in reply to Dong Hui
    TI__Guru**** 393215 points
    What is the silicon revision of the processor on your Starter Kit? SR1.0 processors had higher consumption on the RTC power rail. You can see some measurements here: processors.wiki.ti.com/.../AM335x_Power_Consumption_Summary