Re: gEDA-user: terminators

[DJ Delorie <dj@xxxxxxxxxxx>]

> 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.

This is a proof-of-concept board, so EMI isn't an issue (yet).
Certainly, adding the analyzer's probes will add EMI all over the
place too.

> 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.

Series resistors on the back (stub) side, maybe?  Near the via?

> 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.

The problem is, the connector is somewhat pre-defined for me - I'm
using old hard drive cables as the LA's connector is a standard 2x20
ribbon cable connector.  And, my software doesn't let me rearrange the
signals in a "bus" so I had to bring them out in the right order.

> > 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?

Hmmm... yes, it does - it has both slew rate control and drive current
control.


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