We have a PIC18F8722 connected to a USB bridge chip, specifically the
Exar XR21V1410.  Pretty simple...USB in, TTL serial out and pretty cheap
(in volume).  The PIC responds with a string of character when it gets a
specific (and what it considers illegal) character sent.  We use this as
a ping to the device from our PC application to know if the target is
there and ready to run.

Running under Win7 64 bit, we have an issue where sometimes, and it
varies from PC to PC (some respond better than others) then USB side
simply does not respond. We can do a connect...send...get
back...disconnect several times, and then it just simply stops
responding and wont until a power cycle of the bridge.  I've verified
that the PIC does send the data (captured with a logic analyzer and
decodes), and using a USB protocol analyzer that the data isn't
transmitted.

I have also seen this with the SiLabs bridge and the FTDI  (still
working on getting the Microchip bridge to enumerate correctly).  So I
am not sure it's the hardware side yet, but still open that it is.  I've
been working with the factory folks on this...but duplicating it is
being an issue as well.

To automate a test, we have a script running in a terminal program to
connect, send, get back and disconnect until failure.  Looking at the
USB side, at the control end point, nothing appears different between a
good and bad sequence.

Running on XP 32bit, XP64 bit appears to not fail at all.  Not sure on
win7 32 bit, but I believe we have seen failures on this as well,
primarily been concentrating on 7-64 as we have a machine that shows
this problem more frequently.

So in order for the transmit end point to fail...what could be causing
this?   Does the slave (exar bridge) have to tell the host that it has
data ready to transmit?  And wont unless the host ack's it?  Note also
that the bridge is NOT dead.  I can send data from the PC and it still
appears at the PIC, and the PIC responds each time.  So the control and
receive end points are still active.


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