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Date: Mon, 27 Jul 2015 16:33:07 +0200
From: "Gabriel Paubert (paubert AT iram DOT es) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Subject: Re: [geda-user] high freq, trace lengths (was: pcb-rnd feature poll:
 please vote)
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On Mon, Jul 27, 2015 at 02:09:52PM +0200, gedau AT igor2 DOT repo DOT hu wrote:
> 
> 
> On Mon, 27 Jul 2015, Gabriel Paubert (paubert AT iram DOT es) [via geda-user AT delorie DOT com] wrote:
> 
> Thanks for the details!

Well, these details correspond to the kind of things that I do, which
does not civer al the use cases. 

> 
> I have one question about stubs.
> 
> >In both cases:
> >- these are point to point signals and stubs are essentially prohibited,
> 
> http://nxp.com/documents/application_note/AN10935.pdf
> 
> fig 4 on page 9 shows how two DDR rams connected to a single MCU.
> There are regular T junctions there. I guess with proper trace
> lengths/impedances/termination this should be possible.

The schematics do not necessarily reflect the implementation. The
chips may be put next to each other with the control signals routed
alternately routed between the pins or on another layer with very
short stubs to the chip.

I've never done a DRAM bus. My domain is more analog/mixed signal RF
and microwave designs, which is simpler (fewer signals, and no . 


> 
> I've seen such topology (in application notes) before, that made me
> think the best would be to calculate lengths independently on all
> the three arms of the T. Does this idea make sense?

I'm not sure. There is a length below which you can assume a short
stub length is a lumped capacitance.

    Regards,
    Gabriel