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USB usage with <3.3V supply

crypton

Hei, I couldn't really find a question regarding this use-case.

I'm hoping to interface a USB (2.0) charging capability to my design and thus I need to have a communication to a USB capable uC, which in this case is a 2.8V (or generalize under 3.3V) powered nRF52840.

Looking in the Product Specification, VDD can be from 1.7V to 3,6V, but USB DP and DM lines are specced from 3.0 - 3.6V. I'm currently understanding that attaching 5V bus voltage to the '<3.3V nRF's' VBUS pin is OK.

So basically:

- am I good, interfacing USB communication lines to a <3.3V powered nRF ?

- Or do I need some form of level shifting between USB from host and <3.3V powered nRF ?

- Then also I'm planning to implement placeholders for pull-up resistors on the DM and DP signals - depending on which criteria applies in previous questions, how would it be good to pull-up these resistors to? Could DECUSB be used for that or directly to VDD is safe as well ?)

  • +1

    Hi,

     

    First, the nRF52840 main supply is independent from the USB peripheral. The USB peripheral can be used with supply across the entire valid supply voltage range, in both 'normal voltage configuration' and 'high voltage configuration'. The USB peripheral is to run off the USB bus voltage, VBUS between 4.35-5.5 V for all main supply voltages.

     

    You may either supply the nRF52840 on VDD ('normal voltage', VDD <3.6 V) or on VDDH, 2.5V < VDDH < 5.5 V). Supplying on VDDH will allow you to use the 5V VBUS as supply source without an external regulator, the nRF52840 will run off the downconverted VDD, which is configurable 1.8V - 3.3V. See the PS for more. If you plan to have the device operate also when the USB cable is not connected, you will need some externals to make sure you switch between the power sources as you need.

    - Yes, the USB interface works for <3.3 V powered nRF, just make sure you hook up the main supply correctly (described in the Product Specification).

    - No level shifting is required, you may route the lines/bus directly to their respective target on the nRF52840.

    - No pull is needed on D+ or D-. DECUSB is to house a 4.7µF cap to ground, and should not be used for anything else.

     

    Best regards,

    Andreas

  • 0 in reply to Andreas

    Thanks for clearing things up!

    I'm going to run nRF in Normal mode, VDD and VDDH connected together to another power rail at 2.8V. (just for clarification, I don't power nRF directly from USB (but indirectly), I need the USB only for communications.)

    The pull-ups on D+ D- was for (as I have understood) defining USB 2.0 device's speed - Full or Low speed (but another topic in itself I'm sure). As I haven't done this on nRF before, I figured having placeholders would be a nice addition for the prototype.

    So just to add - I'm safe to assume that When VBUS is powered, the D+ and D- are at their suggested power levels (3.3V compatible), provided by the nRF? Meaning,the type/speed of the device can be manipulated from nRF (internal pull-ups?) and no external components are needed ?

    Otherwise I don't see any way to have pull-ups (to VBUS) without level shifting (or voltage division/clamping), cause VBUS is a bit too high for D+ and D-.

  • 0 in reply to crypton

    Hi,

     

    The nRF52840 USBD peripheral supports full-speed (12 Mbps) only, the pull-up behaviour is described in the PS. When VBUS is powered, D+ and D- are regulated by an LDO internally to 3.3V. No external components are needed, though you might want external ESD protection diodes.

     

    Best regards,

    Andreas

  • 0 in reply to Andreas

    Missed that :/

    Thank you very much for references.

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