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BQ4802LY: BQ4802LYDW not increasing time

Chris Schneider
Prodigy 130 points
Other Parts Discussed in Thread: BQ4802LY

I have a circuit I have designed around a BQ4802LYDW.  I have hooked up the 5k resistor between the RST and the VCC pin as required as well as a battery to the BC line (positive side).  I also have a 32.768kHz crystal between X1 and X2 as required.

I can set the clock but when I check the time it is the same as what I set it to.  For example, if I set it to 22:08:00 and then come back an hour later it still shows 22:08:00

I have made sure there are not solder bridges, replaced the crystal, verified correct voltage supplied to the clock chip as well as correct voltage on the battery.

Any suggestions of what could be the culprit?  It feels like a crystal or a resistor issue but for the life of me I cannot figure it out.

The below is a basic diagram of how I am powering the BQ4802LYDW.  I am using the BQ4802 VCOUT (PIN #1) to power my SRAM.

Thanks,

Chris

  • 0
    Prodigy 130 points

    i have measured the resistance between the RST and the VCC pin with the 5k Ω in place and i get about 1.5k instead of the 5k. 

    I have noticed also that if the battery is not powering the BQ4802LY RTC then the resistor value reads 5k, but the moment I put in the battery it drops to 1.5k between the RST and VCC pin.

    I have used this same circuit it other designs and do not have the issue.

    I'm wondering if this is the problem??

  • 0
    Prodigy 130 points

    I am wondering if the crystal requirements in the datasheet have any type of tolerance.  Like the Series Resistance calls for 45kΩ, however I can only find 35k and 40kΩ, will any of these work, if so any ohm value better to use than the other if I cannot locate the 45k? I have tried a few different manufacturers of a 32.768kHz, 6pf, 35k ohms crystals and all have the same effect, the chip is not recognized and does not cycle time.  Whatever I set the clock to stays at that time and never increments.

    I have one unit that works with a different crystal I used 2 years ago and is no longer made on the same board design.

  • 0
    Prodigy 130 points

    I guess no TI Techs want's to follow up on this?

  • 0 in reply to Chris Schneider
    TI__Mastermind 42070 points

    Hello Chris,

    Apologies for the long wait on this. This is an older device with limited support/knowledge on it. The exact group this product falls on is not really well defined at the moment. But I don't want to keep you waiting, so I will try to help as much as I can! 

    I did notice that you opened a similar thread on a while back. Was there a resolution in it? It seemed like it went to e-mail.

    Which crystal are you using for this? Do you have a part number? Was it a DT-26 crystal?

    I would check if you are meeting the power-up sequence.

    Best Regards,

    Luis Hernandez Salomon