Powering nrf52840 for USB application with a data only cable

Hi,

I want to have a nrf52840 communicating with a Raspberry Pi over USB. The single USB port of Raspberry Pi 3A+ is also used by a LTE modem. So I have an USB hub IC to handle both.

The board has +5V and +3.3V available. I want to use a USB cable with only the data (D+/D-) lines.

Is it possible to power the nrf52840 like this :

  • VDD shorted with VDDH with 3.3V
  • VBUS pin supplied with direct 5V

The VBUS voltage would not come from the VBUS line of the USB cable as there's none in the cable used.

It would be like "Circuit configuration no. 3 schematic" for nrf52840-QIAA.

Feeding directly 5V to the VBUS pin of nrf52840 would avoid buying a chip like a TPS2042 just for it because I already feed the LTE modem directly in another board and not via the VBUS line of the cable.

  • Hi 

    Feeding 5V to VBUS from some other source should work fine, but I am not sure a USB cable with only the D+ and D- lines will work unless it also includes ground...

    Can you confirm whether or not ground is included in the cable?

    Best regards
    Torbjørn

  • Thanks.

    Actually, the cable has +5V/D+/D-/GND in it but I don't want to use the +5V of the cable due to power limitation. I won't route VBUS of the cable in layout and I will power my 2 USB devices from on-board PSU/LDO. That's why I want to be sure I can feed VBUS the way I was talking about in my first post.

  • Also. Everything is on the same board with a common ground. We use an USB cable juste because we haven't migrated yet to "CMx" models from Raspberry.

    A slightly unrelated question. One of the first prototypes had VDDH left floating but everything seems to work fine. What could go wrong if this pin is left floating instead of shorted to VDD? I don't use a nrf52840 but a BT840X from Fanstel. So maybe VDD and VDDH are shorted inside but I don't think so as they expose both pins on their BT840X.

  • Hi

    Good to hear you have ground available, then what you described should work fine. 

    Carton_ said:
    What could go wrong if this pin is left floating instead of shorted to VDD? I don't use a nrf52840 but a BT840X from Fanstel. So maybe VDD and VDDH are shorted inside but I don't think so as they expose both pins on their BT840X.

    We haven't really tested this configuration. It might work OK, but it could also have impacts on things like power consumption and reliability, so I would strongly recommend keeping them connected. 

    If both pins are exposed on the module it doesn't make sense that they would be connected internally, then you wouldn't be able to use the high voltage mode at all. 

    Best regards
    Torbjørn

  • If both pins are exposed on the module it doesn't make sense that they would be connected internally, then you wouldn't be able to use the high voltage mode at all. 

    Yes that's exactly what I thought.

    We haven't really tested this configuration. It might work OK, but it could also have impacts on things like power consumption and reliability, so I would strongly recommend keeping them connected. 

    I will do so. Thanks a lot for your answer.

    I've contacted them also about this VDD/VDDH topic but they keep telling me I should let VDDH floating in normal mode. I prefer to follow your design guidelines and your advice. Seems more reasonable.

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