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Do OTG plug USB0_ID to VBUS or to DGND?

Paul Singh
Expert 2750 points
Other Parts Discussed in Thread: OMAPL138

Hi,

We would like to ask about the mechanism of USB OTG device recognition:

The attached picture is taken from an evaluation board of TI’s OMAPL138 processor, and the header shown is a USB Mini-AB receiver. The 2nd pin from the right in the upper sub-image is connected to processor’s USB0_ID pin. According to USB standard:

(1)  A peripheral would draw this USB0_ID to GND, and this would notify the evaluation board’s USB controller to assume the role of host and hence drive VBUS.

(2)  A host would drive USB0_ID high, notifying the evaluation board’s USB controller to act as peripheral.

We do not currently have a USB Mini-B header (and do not have OTG host device). We guess that:

For peripheral device to achieve (1) above, the peripheral device needs to tie USB0_ID and DGND together.

What we actually found is that this USB0_ID↔DGND is done by the Mini-A plug.

1.    When Mini-A plug is NOT plugged into the Mini-AB receiver, the resistance between USB0_ID and DGND is 192kΩ.

2.After Mini-A plug is plugged, the resistance between USB0_ID and DGND is 192kΩ becomes 0.

From this we know that USB0_ID and DGND pins must be tied internally within the Mini-A plug.

 

For external host to achieve (2) above, it seems that the corresponding Mini-B plug must NOT tie USB0_ID and DGND. Instead, it will tie USB0_ID to VBUS which the connected external host is driving.

Is this guess correct? We need to confirm this before making the board because we have no B device/plug to make the test.

Additionally, could someone refer me to the official USB documentation describing the electrical connection within A and B OTG plugs? I have read several documents in USG.org but didn’t find one giving such information.

 

Paul

  • 0
    TI__Guru**** 176571 points

    Paul

    For info on cable, please refer to Universal Serial Bus Micro-USB Cables and Connectors Specification, revision 1.01. The original USB On-The-Go standard introduced a plug receptacle called mini-AB which was replaced by the micro-AB in later revisions (Revision 1.4 onwards).

    The mini-A plug has the ID pin grounded while the ID in the mini-B plug was floating. Base on ID status, this is how devices on both end of cable determine who become the host, and who become the peripheral.

    Is your application going to be a peripheral, a host, or OTG? For pure peripheral application, you can leave USB_ID floating. For pure host application, you can tie USB_ID to GND.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Expert 2750 points

    David,

    "The mini-A plug has the ID pin grounded while the ID in the mini-B plug was floating. Base on ID status, this is how devices on both end of cable determine who become the host, and who become the peripheral."

    You are right. Thanks!

     

    Paul