Re: gEDA-user: General ground/power plane design questions

[Randall Nortman <geda-lists@wonderclown.org>]

On Wed, Oct 20, 2004 at 06:49:33PM -0400, Dan McMahill wrote:
> On Wed, Oct 20, 2004 at 09:41:40AM -0400, Randall Nortman wrote:
[...]
> > 2) I would like to be able to extend the SPI and I2C buses between
> >    stacked boards for short distances (perhaps three boards stacked
> >    1.5" apart with about 4" total cable length).  I'm thinking that
> >    flat ribbon cable with every other conductor attached to ground
> >    should provide enough crosstalk dampening for such short distances.
> >    Does this seem reasonable, or am I going to have reflection
> >    problems?  (The boards themselves will have traces up to about 5"
> >    long carrying these signals, which also needs to be considered.)
> 
> I'd just put 100 ohms or so in series with the drivers.  At the speeds
> you're running at for the SPI and I2C, you can afford to slow down the
> edges enough to make your cable more or less look like a lumped circuit.

There are several drivers on each bus (other than the SPI master-out
line, which has one driver and multiple receivers).  I2C is
bi-directional, and SPI puts multiple slaves on the same line.  Do you
advise a resistor in front of each driver?  I assume you advise this
in addition to the pull-up resistors on the I2C lines, right?

Thanks,

Randall