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AM335X USB device (B connector) wiring

Bruno Bittner
Prodigy 235 points

We will be using a dedicated device USB B style connector for USB0 in our 335X based device.  I haven't found any wiring examples for this, but from my reading of the TRM, device mode can be forced by HW by tying high the USB0_ID  (ZCZ ball P16).  Is this correct?  Also, how would we deal with power coming in on Pin1 of the B connector?

Also, what would be the recommended solution for ESD given that we are not OTG/portable/charging/drawing power?

  • 0
    TI__Guru 58315 points

    Based on your forum post it sounds like you are developing an AM335x based product that needs to connect to a host PC.  The product is not an OTG or portable product that requires battery charging and you do not plan to power the product from the host PC.

    If that is the case, you still need to connect the VBUS pin of the USB series-B connector to the respective VBUS pin of AM335x.  This is needed because the USB controller monitors the VBUS voltage to determine is a valid VBUS voltage is present. I also recommend connecting a 1uF – 4.7uF decoupling capacitor to the VBUS.

    You should never apply any external source to the AM335x USB-ID pin.  This pin should only be connected to ground for USB host operation or open-circuit for USB peripheral operation.  I searched the TRM and did not find a reference to pull-up the USB_ID pin.  Please tell me where you read a recommendation to pull-up the USB-ID pin, because this needs to be changed since an external pull-up may damage the device.

    I do not have much experience with system level ESD protection, but have a couple of recommendations.

    Make sure the ESD protection device you use does not insert more than 3 ohms of series resistance on the USB data signals.  The internal USB PHY does not expect any external series resistance on the data signals.

    Many USB ESD protection devices clamp the ESD undershoots to ground and overshoots to VBUS.  The energy from an ESD overshoot on the USB data signals will get dumped into VBUS through the ESD device.  Therefore, it is important for VBUS to be tightly coupled to ground.  Products similar to yours do not commonly connect VBUS to a power plane since it is only used as a voltage reference signal.  I have seen cases where a simple decoupling capacitor near the ESD device does not provide a good high frequency coupling to ground.  This is very important since the ESD overshoot needs to be shunted to ground through the ESD device and VBUS capacitor.  If possible, assign a power plane or portion of a split power plane that is adjacent to a ground plane to VBUS which will create a good low inductance high frequency capacitor.

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 235 points

    peaves said:

    You should never apply any external source to the AM335x USB-ID pin.  This pin should only be connected to ground for USB host operation or open-circuit for USB peripheral operation.  I searched the TRM and did not find a reference to pull-up the USB_ID pin.  Please tell me where you read a recommendation to pull-up the USB-ID pin, because this needs to be changed since an external pull-up may damage the device.

    The USB-ID pull-up is my misinterpretation of the relation of the IDDIG register bit vs. iddig internal signal vs. USB-ID pin.  The doc never states to pull-up USB-ID, it explicitly says to leave it floating or pull-down.  Page 2056 SPRUH73– July 2011 section 15.3.7 USB Controller Host and Peripheral Modes Operation.

    Thanks for the detailed description and helpful leads!