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TDA2E: RTC section design with coin cell

SANTHA KUMAR A
Prodigy 100 points

Part Number: TDA2E

Hi 

I'm trying to design RTC section on TDA2E with coin cell battery.

Could someone suggest me how it can be implemented, i'm going through the datasheet and TRM.

But couldn't get enough details like through which pin coin cell power to be sourced (at what voltage) to keep time, date info. and functionality of rtc_iso signal and current consumption details.

Pls respond ASAP.

-Santhakumar A

  • 0
    TI__Mastermind 25170 points
    HI,
    Regarding ISO signal:

    External control of isolation cells. This signal must be
    kept low (0) during RTC mode and high (1) after the
    device power supplies ramped-up. This can typically be
    achieved by connecting rtc_iso to the porz (not rtc_porz)
    with appropriate voltage level translation if necessary.

    I will come back to you for the other questions.
  • 0 in reply to Stanislav Stilyanov
    TI__Mastermind 25170 points
    Hi Santha,
    I didn't find coin cell in any of the EVM-s. I think this was not validated. If an RTC was to be used, most designs implemented off-chip RTC w/ or w/out coin cell.
    Can you tell me more about your design? Do you need just to keep time during flat/disconnected car battery?

    Regards,
    Stan
  • 0 in reply to Stanislav Stilyanov
    Prodigy 100 points
    Dear Stanislav,

    Yeah, I want to keep time during disconnected battery, As you suggested i can use off-chip RTC but we want to check On-chip RTC since our PCB has placement constraints.

    Regarding rtc_iso signal, can you tell me what it does during RTC mode (keeping signal LOW) and not RTC mode (Keeping signal HIGH)

    -Santhakumar A.
  • 0 in reply to SANTHA KUMAR A
    TI__Mastermind 25170 points
    Second question first. During RTC mode, the whole chip (SoC) is shut down except for the RTC module. The connections of RTC with the rest of SoC must be isolated to prevent from unnecessary current flowing to shut-down domains. The rtc_iso pin simply enables this isolation.

    About the RTC powered with coin cell, this is not how the RTC was integrated.
    I mean, the RTC needs 3 separate power supplies:
    - digital core at 1V
    - I/O at 1,8/3,3V
    - 32k oscillator at 1,8v
    As you can see, you cannot simply add a coin battery of 3-V or 1,5-V. You would need a bunch of circuitry to make the all.
    Also, I did not find support for coin cells in the PMICs.
    The only option I see is if you add a 12-V accu (backup) in front of all power convertors, but I'm sure you don't want that.

    My very personal opinion is that keeping time of day during car battery disconnect is almost (why not say completely) unneeded.
    Car battery has to be disconnected only during certain major car repairs that even don't occur every year.
    And, in rare conditions when battery becomes flat to start the car, it will still have enough voltage to keep RTC running.
    And most important! If you don't have other reliable source for time of the day ( I believe you don't) driver will have to tune the clock typically once for 3 months. I'm sure the clock will drift 1 to several minutes for that period.
    That is, one way or another driver will have to synchronize the clock more frequently than after battery disconnects...

    You might have other considerations to keep the time ticking, but if you don't, then I think it would be very expensive feature.

    Regards,
    Stan
  • 0 in reply to Stanislav Stilyanov
    Prodigy 100 points

    Dear Stan,

    Thanks for your reply.

    from your reply i understood that it is not optimum to use On-chip RTC with coin cell, and may not be required,

    or else better i can have Off-chip RTC if still i want one.

    Thanks a lot for your answer.

    I will come back if i have any other queries.

    Regards,

    Santhakumar A.