BQ4802Y: BQ4802Y don't keep clocking in VBC mode

CHEN YOUNG WU

Part Number: BQ4802Y

I've use BQ4802Y as system real time clock. VCC is 5VDC, VBC=3VDC Li Battery. I set the /STOP=1 in control register(E) and ABE=1 in Alarm enable register(C). I've checked that Control Register(E) content=06h, Flags Register(D) content= 01h, Alarm enable Register(C) Content=09h. when power by VCC the BQ4802Y is working correctly but when power by VBC clock is not keep clocking. So when BQ4802Y be power by VCC again, the clock is start to clock from at VCC off. I use VOUT to power SRAM and found that SRAM content are retained correctly. I don't known what's wrong about above mentioned problems ? I already try couple of weeks on this problem. I hope someone can help me to solve it. Kind regards; Marshall Wu 2019/04/14

over 5 years ago

0 CHEN YOUNG WU over 5 years ago

Intellectual 540 points

0 Rob Rodrigues over 5 years ago in reply to CHEN YOUNG WU

TI__Mastermind 19780 points

Hi Chen,

When VCC is removed, are you receiving the correct flags from the bq4802Y? The bq4802Y should bring /CEout to Vohb level.

Is the slew rate of VCC falling meeting the 300us requirement? See tF in the datasheet power-down/power-up timing.

Can you probe the crystal with an active FET probe with 1.5pF load or less when VCC = 0V? Is the crystal still oscillating at this time?

Are the capacitors C31,C32 populated or removed? Can you try removing them if they are populated?

Best Regards,

0 CHEN YOUNG WU over 5 years ago in reply to Rob Rodrigues

Intellectual 540 points

Hi Rob: Thank you very much for your solution. I removed C31 and C32 then problem is solved.

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BQ4802Y: BQ4802Y don't keep clocking in VBC mode