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Battery - backed in case of arbitrary power loss conditon???

Tu Nguyen Ngoc
Prodigy 90 points

Hi everyone. I am working with tm4c1294 board. I need to interface my board with a RTC module which have a external battery to prevent losing data in case of arbitrary power loss. I found that tm4c have hibernate module with built-in RTC. So i think i have 2 solutions for my project.
1. Use RTC of hibernation module and connect battery with Vbat pin (which i really confuse that how battery will work in paralle with Vcc source, and after drop Vcc)

2. Use ds1307 and ofcouse a external battery for it.
Shoud i use first or second solution? Can someone suggest me any idea?

Thanks.

  • 0
    TI__Guru**** 244400 points
    Hello Tu

    You can use the option #1. In this case the VDD to the VBAT pin must be removed by removing the R39 resistor and connecting the Vbat supply at TP11 to a coin cell or separate 3.3V supply.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Guru 47900 points

    Hi Amit,

    My group has no experience w/xx129 - yet I recall many MCUs being "happier" when their battery/back-up power voltage was less than their main (VDD) power voltage.   A coin cell @ ~3V0 thus qualifies - but a "separate 3V3 supply" may not. (and what if the separate supply voltage should "exceed" that of VDD?   One horror may be that the "back-up" battery "runs the show" until it discharges to drop below the "barely used" main supply...shortening battery life.)
     
    We've encountered such issues @ clients - perhaps xx129 has "escaped" such concern - I offer this detail as broader perspective... (just in case)

  • 0 in reply to cb1
    TI__Guru**** 244400 points
    Hello cb1.

    The separate 3V3 supply can be a 3V0 supply as well on the TM4C129x.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Guru 47900 points
    Hi Amit,

    Indeed that's as expected.

    That said - does the xx129 prevent, "switch-over from VDD to (assumed) Battery" - if & when - V_Bat exceeds VDD? (IMO - that's undesired - we'd like the switch-over [to battery] (only) as VDD decays below a known (MCU operational) voltage level - and the opposite to occur as VDD restores.)

    I'd like to refer a particular client to your xx129 - yet we've no xx129 experience - and the switch-over performance (as I've outlined) will likely prove of value to o.p. (and others, here) as well... Thank you for your time/attention...
  • 0 in reply to cb1
    TI__Guru**** 244400 points
    Hello cb1,

    It is the other way. If VDD > VBAT then the supply to the Hibernate is from VDD. This behavior of the hibernate has not changed.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Guru 27665 points
    Amit, I think cb1's point was that if VBAT > VDD (say VBAT = 3V3 and VDD = 3V2, VDD well within spec) you risk draining the battery. It is therefore better to run with a lower voltage cell to avoid prematurely draining the battery.

    Robert

    Or alternately a supercap presumably.
  • 0 in reply to Robert Adsett
    TI__Guru**** 244400 points
    Hello Robert,

    Yes, that is correct. My apologies, should have been careful with the 3V3 comment.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Guru 47900 points
    Thank you both - now we have better understanding...
  • 0 in reply to cb1
    Prodigy 90 points
    Dear all,
    Thank you for your time and your attention, after reading all of your replies i think now i have better understanding as cb1 said :). I have some question, is the tm4c automatically using batterry when VDD suddenly drop out? Do i need some kind of circuit for detect/switch to battery source? Cause i saw some of them with other MCUs.
    Thanks.
  • 0 in reply to Tu Nguyen Ngoc
    TI__Guru**** 244400 points
    Hello Tu,

    No. The Switchover is handled inside the micro. Make sure that the VBAT is always present and the Hibernate module configured to wakeup the device if the low voltage is detected on the VBAT.

    Regards
    Ami
  • 0 in reply to Amit Ashara
    Guru 47900 points
    May I note that having such "power-source switchover" - handled automatically & correctly - via the MCU - is a very BIG Deal! Some here recall the olde dayze - when multiple components had to be carefully selected - and (even then) the switchover was not noted for being both robust and, "Loss-Free!"