Laying out Ethernet

autodeskguest over 16 years ago

Hi all,

I would like -- probably quite stupidly -- to ask if anyone has built a

working Ethernet device using only a dual-layer board. All the appnotes

seem to assume you have multiple power and ground planes, which pretty

much assumes a four-layer board or more. I'm assuming an integrated

MAC-PHY chip and a magjack here, so at least the number of spans on the

board is only one, as opposed to three.

It would be good to hear how critical placement is and if there is any

particularly good/bad ways to do it.

Additionally, most modern silicon is available only in very small

packages (typically 0.5 mm TQFP and so on.) Anyone has any suggestion

for how best to deal with the change from an inch-based grid (for the

larger components) to a metric grid (for the small ones)? This has to

fit into a Eurocard chassis, completely with a B/AB connector, so using

an all-metric grid is unfortunately not possible (although "everything

except the connector is on a metric grid", of course, is.)

Many thanks,

-hpa

autodeskguest over 16 years ago

On Wed, 10 Dec 2008 17:30:04 -0800, H. Peter Anvin wrote:

Hi all,

I would like -- probably quite stupidly -- to ask if anyone has built a
working Ethernet device using only a dual-layer board. All the appnotes
seem to assume you have multiple power and ground planes, which pretty
much assumes a four-layer board or more. I'm assuming an integrated
MAC-PHY chip and a magjack here, so at least the number of spans on the
board is only one, as opposed to three.

It would be good to hear how critical placement is and if there is any
particularly good/bad ways to do it.

Additionally, most modern silicon is available only in very small
packages (typically 0.5 mm TQFP and so on.) Anyone has any suggestion
for how best to deal with the change from an inch-based grid (for the
larger components) to a metric grid (for the small ones)? This has to
fit into a Eurocard chassis, completely with a B/AB connector, so using
an all-metric grid is unfortunately not possible (although "everything
except the connector is on a metric grid", of course, is.)

Many thanks,

-hpa

You might want to have a look at the XPort and/or the WIZnet devices.
Both have plain-vanilla through-hole mounting, and provide both PHY and
MAC layers. All the necessary smarts are built into the sockets
themselves.

As for mixed English/metric boards, my suggestion would be to position
the English parts and lock them down, then switch to metric and position
those parts. As has been mentioned here before, when laying out boards
with differently-gridded parts, the best bet is to do the layout from the
coarsest part until you're almost ready to make the final connection,
then finish the layout from the finely-gridded part to the endpoint of
the track as indicated by the high-quality graphic below

----
X
--
X<[E]

Hope this helps...

Dave Merriman
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