[Gumstix-users] USB: MAX6348 or MAX823

[Joshua F. <jm...@ka...>]

Hi all,

So looking at the USB design of the buddyboard (or daughterboard, which do you
prefer?) a couple of us on the #gumstix list today (by the way if you're not on
the list [and most of you aren't :0] you're missing out) noticed that the
PXA255 design manual recommensds the following:

The 5 to 3.3 voltage divider is required since the device GPIO pins cannot
exceed 3.3 V. This voltage divider can be implemented in a number of ways. The
most robust and expensive solution is to use a MAX6348 Power-On-Reset device. 
This solution produces a very clean signal edge and minimizes signal bounce.
The more inexpensive solution is to use a 3.3V line buffer with 5 V tolerant
inputs. This solution does not reduce signal bounce, so software must
compensate by reading the GPIO signal after it stabilizes. A third solution is
to implement a signal bounce minimization circuit that is 5 V tolerant, but
produces a 3.3 V signal to the GPIO pin.[1]

Looking at the skematic for the daughterboard it looks like the MAX823 chip is
substituted in place of the MAX6348.[2]  We concluded that there are some
potential problems with that design (specifically, it looks like the PXA's
input port is being driven above its own 3.3V Vcc).  Any comments?

[1]. Section 4.1, p. 55:
ftp://download.intel.com/design/pca/applicationsprocessors/manuals/27869401.pdf

[2]. http://www.gumstix.com/docs/schematic.jpg


--
Joshua Ferraro
http://kados.org