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BQ25886: Sharing USB data lines with other device

Part Number: BQ25886
Other Parts Discussed in Thread: TS3USB3000

Hi,
we have a custom board with a BQ25886 and an AM335x that are sharing the same USB connector (Similar setup as e2e.ti.com/.../bq25886-how-to-use-bq25886-and-usb-audio-codec-with-shared-usb-connector). The USB interface is intended to work as DRD port and at the same time power input to charge the battery.

Regarding the D1/D- USB signals we have found two suggestions here in the forum and in the data sheet. We have tried both and it appears as if neither one does work for out setup:

Option 1: Shorting D+/D- pins of the bq25886 and rely on the input current optimizer (ICO): This works well for charging the battery and with most USB host ports BUT there are host ports (like e.g. the Raspberry Pi) that report an over-current error after the ICO algorithm, then switch VBUS off and on again and then the cycle starts all over again. 

Option 2: Connecting D+/D- pins of the USB connector to both the bq25886 and to the processor: With that option we observe another issue: We want to be able to charge the battery when our device (handheld test equipment) is switch off. In this state the AM335x is not powered, so the D+/D- lines of the AM335x are clamped to 0V by the ESD protection diodes of pulled down and this appears to interfere with the USB BC 1.2 detection. The result is, that the charging current is limited to 500mA even though the power supply is capable of delivering 3A.

Does the bq25886 have a status output that indicates when the USB BC 1.2 detection is completed so that we could control a USB multiplexer with that output? Is there a USB data repeater with a high-z output, so that we could disconnect the AM335x when the processor is not powered? Any other ideas what we could do with the D+/D- signals to solve above issues? 

Thank you for your help!

Best regards
Reto

  • Hello Reto,

    Because this charger is standalone there are not many workarounds for these corner cases. The only status output is the STAT pin and PG pin. I'm not aware of any USB repeaters with high Z outputs, but that may be a option to use when your processor is not active.

    Sincerely,

    Wyatt Keller

  • Hello Wyatt,

    thank you for your quick reply! I have checked the PG status output of the bq25886 and according to the datasheet this pin goes low after the input source detection is completed. If this includes the USB BC1.2 detection, this might be output that I was looking for.

    What do you think about the idea of using this output to control a USB data multiplexer? 
    Can you suggest a specific part? 
    Are there any reference designs ?

    Best regards

    Reto

  • Hello Reto,

    Yes the PG should toggle when all 4 of those conditions are met, the input source detection is described in the datasheet: 8.3.3.2 Input Source Type Detection in figure 13 it shows the process flow chart until detection is complete.

    I think it could work, but I would recommend testing first because I have not seen this application before. I think a chip like the TS3USB3000 would work for this application, but again I would recommend testing,.

    Sincerely,

    Wyatt Keller