FreeLists / si-list / [SI-LIST] Re: USB full-speed (*not* high-speed) layout and stubs.

[steve weir <weirsi@xxxxxxxxxx>]

Andrew, the drivers are required to limit Tr / Tf in full-speed mode to 
no less than 4nS.  The stub that you are introducing should not be any 
consideration as long as you limit it to a couple of inches.  I don't 
see a good reason for the resistors at that slow an edge rate.  If you 
are really worried, put a series damping resistor in each the + and - 
line at the Y divide.  But I really do not see that as necessary.

Steve.
Andrew Morley wrote:
> Hi all,
>
> There are plenty of guidelines for high-speed (480Mb/s) USB PCB
> layout, but I can't find any for full-speed (12Mb/s).  Obviously
> full-speed is much less demanding, and for what I'm considering doing,
> it will need to be.  
>
> I would like to provide a standard USB 'B' socket (full-speed, *NOT*
> high-speed).  That all sounds pretty straightforward. However I'd also
> like to connect the same USB transceiver to a proprietary connector
> (that carries USB and other signals).  The system would be
> (mechanically) arranged so that it would not be possible to use both
> sockets.  With the proprietary connector disconnected, the user would
> be able to use the standard USB; with the proprietary connector mated,
> USB would be carried over the connector and the user will not be able
> to use the standard connector.  It is important that the standard USB
> 'B' port should pass USB compliance.  
>
> What I'm considering doing is having the transceiver connected to each
> port by a set of impedance-matching resistors (i.e. each port has its
> own resistors).
>
> For high-speed (480Mb/s) USB, paralleling-up the two ports in this way
> would be completely unacceptable.  For full-speed (12Mb/s) and below,
> I have been unable to find a reason why this shouldn't work.  However
> I'm still rather uneasy about it.
>
> Has anyone tried this?  Did it work?  Were there any problems
> (especially passing compliance)?  Any thoughts?
>
> I am aware that there are reasonably cheap analog switches that can be
> used for USB - but I'm sure you know how the pressures on BOM cost are.
>
> Many thanks for all the provocative discussion over the last few years,
>
> Andrew
>
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