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USB to USB layout for Davinci to Host communications

Frank Field
Prodigy 220 points

I have been searching for a recommended layout to connect a DM368 to a host processor via USB on the same PCB.  I believe this is similar to running Ethernet over a backplane, no transformers, but have not been able to locate any document(s) that discuss(es) this.  Is there a reference or an example of how to correctly implement this?

 

Thanks,
Frank

  • 0 in reply to Marcus Cooksey
    Prodigy 220 points

    Hi Marcus,

    Thanks but this is not what I am looking for.  We want to connect two processors on the same PCB directly using USB, there are no connectors or transformers.  With this type of signalling there is usually some "conditioning" needed to make it work and that is the guideline we are looking for.

    Thanks,
    Frank

  • 0 in reply to Frank Field
    TI__Intellectual 1670 points

    As far as “conditioning”, I don’t think any would be required.  A USB host to peripheral connection works fine in the logic, as each is thinking it’s talking with an off-board host or peripheral.  For the electrical signals, connecting by a cable would be nearly identical to connecting with a very short cable.  USB is tolerant of nearly any cable length, including no cable (for a thumbdrive), so I don’t see why there would be any extra logic or glue required.

    The DM368 holds to the USB spec, and the USB spec specifies all the compatible voltage levels for any cable length for both the host and the peripheral. 

    I would caution to make sure to do a very good job for the PCB routing.  We have some good USB papers on TI.com (like SPRAAR7, use Google to search for this paper before our TI.com search engine is overhauled on Sept. 1st), or reference the USB.org paper at:

    http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf

    The PCB routing will be important since there will be no cable to smooth out any spikes.  Single ended impedance will be very important for this interface.  I’d shoot for 30-45 ohms.  Differential impedance should be ~90 ohms, but on the PCB this will be less important.