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BQ3285 - Register "D" does not follow battery state

Nuno Jorge Santos Tavares
Prodigy 20 points
Other Parts Discussed in Thread: BQ3285

Hello,

I'm using a BQ3285 RTC chip interfaced with a MCS51 compatible uC. Everything works fine except the value of bit 7 of register 0Dh. According to the datasheet, this bit (VRT) should indicate if the battery's voltage is/isn't above an acceptable value of 2,5V.

But such doesn't happen. Upon power up (VCC) if Vbat>2,5V this bit reads "1" and keeps reading "1" even if the battery is disconnected and BC pin shorted to GND!

The opposite is also true: Upon power up (VCC) if Vbat<2,5V the bit reads "0" and keeps reading that way even if the battery is reconnected.

I'm using a CR2032 Lithium button cell (3,2V when new)

What could be wrong here? I've chosen this chip for this particular feature, because this clock will be working 24/7 for many years. I need to know if and when the battery is dead before the next power fail!

Thanks in advance

Nuno Tavares

  • 0
    Prodigy 20 points

    If this info helps,

    I've tried updating the RTC with invalid dates and times. The chip counting goes nuts incrementing hours beyond 23 and such...

    But bit "VRT", register "D", bit 7, remains unchanged through out all this. Even powering off/on with these wrong hours/dates has no effect on this bit.

    It only follows the battery state upon VCC power up and that's it...

    What is the meaning of the "Valid Ram and Time" name of this bit anyway?

    Nuno Tavares