Re: gEDA-user: terminators

[Ben Jackson <ben@xxxxxxx>]

On Sat, Apr 04, 2009 at 03:29:16PM -0400, DJ Delorie wrote:
> 
> The right half of U2, and U2, run at 133MHz.  The longest trace is the
> CLK line, at just over 3 inches, the shortest is just under an inch.
> However, most of those lines are brought out to logic analyzer
> connectors, which may add up to another 1.8 inches (DQ11, for example,
> has a combined length of 3.9 inches).

I think you'd be fine without the stubs going to the header.  Even with
the stubs your SDRAM will probably work but there will be terrible EMI.
I chased down an EMI problem on a board with ONE stub at 125MHz (due to
leaving a clock testpoint enabled).  You have 20+ stubs.

Ideally you'd put the LA connector in the middle of the bus.  Just pick
a higher density connector.  AMP MICTOR connectors are a popular choice.

> I'm thinking I have enough space to put in some series terminator
> packs (8x 0402 SMT) but where and how big?

Does the FPGA you're using have controllable drive strength?  That alone
is probably enough for your design.  Otherwise put the series resistors
on the driving side.  The value is a bit of a black art, but 27..51R is
in the right ballpark.  To measure their effectiveness beyond pass/fail
you'd need a very fast (6G+) scope.

-- 
Ben Jackson AD7GD
<ben@xxxxxxx>
http://www.ben.com/


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user