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AM3517 USB interface - how to isolate

TommyG
Genius 13045 points
Other Parts Discussed in Thread: AM3517

Need to add operator isolation barrier to high speed USB port.  Does anyone know of any designs that provide 2.5KV isolation of the ULPI interface and allow high speed USB 2.0 operation?

 Currently envisioned design using the ULPI subsystem of the AM3517 processor, parallel 150Mbps isolation barriers and SMSC USB3320 (Highly Integrated Full Featured Hi-Speed USB 2.0 ULPI Transceiver) with minimal glue logic.  There are problems with the timing of control signals (most critically being the STP signal wrt CLK) due to the large propagation delays and propagation delay spread of isolators.

Any help greatly appreciated.

Regards.

  • 0
    TI__Guru 62855 points

    I have never seen this requirement on any AM35x system, so I can not offer any alternative solution.

     

    It is very important you do not violate any of the timing requirements of the ULPI.

     

    I would expect the added delay of the isolation buffers to make it very difficult to meet the transmit data setup time requirement specified by the ULPI USB PHY.  This occurs because the PHY sources the UPLI clock to the USB controller in the AM35x and the combined round trip delay of the clock getting to the USB controller and the transmit data returning to the USB PHY (which includes 2x the buffer delay, 2x the PCB delay, ULPI clock output delay from the PHY, and the ULPI clock to transmit data delay of the AM35x) must be less than the ULPI clock period minus the ULPI USB PHY data setup time.

     

    I do not think the developers of ULPI expected this type of insertion delay.

     

     

    Regards,

    Paul

     

  • 0 in reply to peaves
    TI__Genius 13045 points

    Paul,

    Thanks for the quick response.  I'll marked this as answered.

    Regards.

  • 0 in reply to TommyG
    Prodigy 20 points

    Paul  already explained USB wasn't meant for isolation.

    One thing to remember is D+ and D- are referenced to Ground.

    Analog Devices makes a Full Speed (1.5M and 12M) isolation device. Cool little device,  but does not work for High Speed (480M) communication.

    http://www.analog.com/en/interface/digital-isolators/products/overview/over_iCoupler_USB_Isolation/resources/fca.html

     

    Other way I have seen the isolation is to go to Ethernet, use Ethernet (medical transformers are rated at 2KV),  then back to USB.   Lots of software.   Company in British Columbia sold commercial products, and also 'bare bone modules' that you could implement.   Had a design with 500pcs, needed isolated HS USB, so it was simplier to buy modules from www.icron.com      

    Tom Walter