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

[Daniel Wisehart <dw@yankee.us.com>]

Hello Randall,

First off, let me say that at the frequencies you are dealing with, much of 
what you ask about just doesn't matter.  The processor on our board runs at 
48 Mhz and it also does not have any noise margin problems.  If you were 
instead dealing with USB 2.0 at 480 Mhz, then these factors start to matter.


> Nevertheless, I do intend to separate the analog and digital ground
> and supply planes, while keeping them on the same layer.  That is,
> there will be non-overlapping copper regions for each, connected
> together at one point.  I gather this is pretty standard, although I'm
> not sure how to handle the fact that the ADCs are both analog and
> digital.  I guess I'll just try to guess which parts of the chip are
> doing which based on the pin functions, and separate the planes along
> the best dividing line I can discern.

Broken planes can cause more problems than they solve, depending on how they 
are done.  For the ADC, check the datasheet for a recommended layout.  The 
ADC9826 part we use specificly recommends against splitting the planes below 
the chip.


> 1) What's the best arrangement for the power planes?

A lot of board manufacturers will not be able to put a fairly solid piece of 
metal on the top or bottom layer.  If you try, they will often remove a lot 
of the metal in a grid pattern to prevent problems producing the boards.  
They instead need the solid planes back to back so that they have a balanced 
inner board with about the same amount of copper on it.  Besides, silkscreen 
usually cannot be put on top of metal--because it will not adhere--so you 
would have to do without one.


> 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).

We use 49.9 ohm terminating resistors at the source end to take care of these 
reflections.  When there is bi-directional traffic, the resistors are only 
needed on one end.


Regards,
Daniel