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Original question:

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

AM3352: RTC-Only Mode with TPS65217C

Paul Richards
Prodigy 205 points
Part Number: AM3352
Other Parts Discussed in Thread: TPS65217, TPS65218

Hi,

We have a design incorporating an AM3352 and TPS65217C (with DDR3L) and would like to support the RTC-Only mode. We are considering the possibility of using either the internal RTC, or a discrete I2C device (as recommended for lowest power consumption).

In the case of the internal RTC, the discussion here suggests the only requirement is to supply an independent 1.8v to VDDS_RTC. However, SPRS717J indicates that RTC_PWRONRSTn should be asserted for at least 1 ms before being released. Is this required, or can it also be provided from 1.8V VDD_ RTC? Also, presumably if RTC alarm or cyclic wakeup is required, then PMIC_POWER_EN should be connected to a circuit that can re-enable 5V supply to the PMIC. Also I believe ENZ_KALDO_1P8V and CAP_VDD_RTC should be grounded and connected via a 1uF decoupling capacitor to ground, respectively. Is this correct?

In the case of a discrete RTC IC, I believe the following is required (according to the data sheet, and errata):

  • RTC_PWRONRSTn should be asserted (grounded)
  • RTC_KALDO_ENn should be connected to VDDS_RTC (VLDO1)
  • CAP_VDD_RTC should be connected to VDD_CORE
  • OSC1_IN and OSC_OUT should be disconnected

Is this correct? In addition the discrete RTC presumably needs to separately wake and supply the PMIC when wakeup is required (e.g. RTC alarm). Are there other requirements, e.g. VIO?

Thanks in advance,

Paul

  • 0
    TI__Genius 13567 points

    When using the TPS65217C it is best to start with the connection diagram in the User Guide (Powering the AM335x with the TPS65217x).

    If you plan to keep the AM335x Internal RTC running at all times (when the TPS65217 PMIC is off)

    • VDDS_RTC will need an always on 1.8V supply
    • RTC_PWRONRSTn can use a capacitor and pull-up resistor to VDDS_RTC to achieve the 1ms delay
      • you do not want to release reset while the supply is still ramping -- this will cause undesired behavior
      • you do not want to use the LDO_PGOOD signal from the PMIC
    • RTC_KALDO_ENn should be connected to GND to enable the internal LDO
    • CAP_VDD_RTC would need a 1uF capacitor and nothing else
    • PMIC_POWER_EN is connected to PMIC PWR_EN as usual (this allows for RTC alarm to wake the PMIC and resume operation)

    If you plan to keep the AM335x internal RTC but don't need it at all times (ex: using external RTC in lieu of the internal):

    • follow the TPS65217C guide and make no changes

    Don't worry about Figure 6-5 and Figure 6-6 of the AM335x datasheet because you are using the TPS65217.

  • 0
    TI__Genius 13567 points
    Let me add, the TPS65218 PMIC has dedicated DCDC supplies for VDDS_RTC and CAP_VDD_RTC. It also works with a coin cell battery. Using these DCDC converters is more efficient than the internal LDO for the RTC.
  • 0 in reply to Ahmad_Rashed
    Prodigy 205 points
    Thanks Ahmad,

    That all makes sense. One description I should have added is that during RTC-only mode we plan to disable the 5V supply to the TPS65217C completely, hence thinking to route PMIC_POWER_EN to additional power control circuitry which could re-enable that supply. Do you have any concerns with this?

    We'll take a look at the TPS65218 as well, thanks.

    Paul
  • 0 in reply to Paul Richards
    TI__Genius 13567 points

    That could work. Applying the 5V power to the PMIC is considered a wakeup condition. You will still want the PMIC's PWR_EN pin connected to keep it on.

  • 0 in reply to Ahmad_Rashed
    Prodigy 205 points

    Great, thanks for the assistance.