As always, good thoughts Doug. From my experience, the USB spec lack of error detection at the lower hardware layers has been a constant compliance hassle especially at high speed when an ESD event can represent a significant portion of a 480 MHz bit time.
You also bring up a very important point that few things are truly differential when viewed from the window of a USB high speed signal eye pattern. The majority of products I have consulted on have not made it through without one of the (rather cheap) specialty common mode chokes built specifically for this purpose by folks like TDK and TOKO. I think it's a very interesting convergence of EMC and SI that the chokes help both. Respectfully, Brent DeWitt Westborough, MA > -----Original Message----- > From: Doug Smith [mailto:[email protected]] > Sent: Tuesday, January 05, 2010 9:43 PM > To: [email protected] > Subject: Re: [PSES] ESD Test Failure of Stainless USB Mouse > > In my experience, differential signaling is of little help for ESD > problems because of the limited common mode voltage range of the > receiver. ESD can generate a hundred volts across a small imperfection > in the way a shield is connected and this shows up as a common mode > voltage at the receiver. > > Because small parasitics can have a large effect with ESD problems, it > is difficult to give meaningful advice on the problem without having > detailed knowledge of the design of the device. > > It is a shame that the USB spec (probably 1.1 here) is not tolerant of > transients. It would have been really easy to include this in the > specification originally and really easy to design to such a spec, > especially for something slow such as a mouse. USB devices seem to > regularly disappear even without ESD. Every now and then I have to > reconnect a mouse or keyboard or other USB peripheral (on several > different computers) because USB does not retry if no answer on the > first one (may have been fixed in later USB specs, but I doubt it). I > have clients that complain about this "feature" of USB. To prevent USB > drops from ESD you generally need a very high quality cable and > connector combination and the cable shield has to be closely referenced > to the signal ground of the receiver to prevent common mode overload > problems. > > I have never liked USB hard drives because of the problems I have > experienced with USB. Firewire, in my experience, has been much more > robust. USB also requires CPU cycles and can slow down if the CPU is > busy doing other things. Firewire does not require intervention by the > CPU which part of the reason that Firewire 400 is faster than USB > "480." > > Doug > > On 1/5/10 2:07 PM, John Woodgate wrote: > > In message <[email protected]>, dated Tue, 5 Jan 2010, > Fred > > Townsend <[email protected]> writes: > > > >> If the discharge is interrupting the signal then the signal low and > >> the shield ground are being combined at some point. USB is a > >> differential signal. Sounds like something is connected wrong at > >> either the mouse end or the USB receiver chip end. > > > > Whatever voltage is on the shield is ALSO on the differential pair, > as > > a common-mode signal. The CMRR may well not be enough to cope, and > the > > permissible common-mode voltage may be exceeded anyway. > > -- > ------------------------------------------------------- > ___ _ Doug Smith > \ / ) P.O. Box 1457 > ========= Los Gatos, CA 95031-1457 > _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 > / /\ \ ] / /\ \ Mobile: 408-858-4528 > | q-----( ) | o | Email: [email protected] > \ _ / ] \ _ / Website: http://www.dsmith.org > ------------------------------------------------------- > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc- > pstc discussion list. To post a message to the list, send your e-mail > to <[email protected]> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to > that URL. > > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
