Board considerations using 2 x L1 64 with ethernet

Technical discussions around xCORE processors (e.g. General Purpose (L/G), xCORE-USB, xCORE-Analog, xCORE-XA).
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Board considerations using 2 x L1 64 with ethernet

Postby Matt » Thu Mar 03, 2011 1:10 pm

Hey Guys,

I am in the middle of designing a board comprising of two L1 64pin chips with ethernet + extras . I intend on joining the L1's with a 5bit xlink. I can not find an example to see what wiring is required between the two devices? I realise the 10 pins for the link connect to the 10 pins on the other chip and from what I can see from the XK1 the debug line is required. Is there any resistors, IO's or any other components required to connect them together?
I also intend on sharing the 25Mhz crystal from the ethernet controller with the two L1's. I have read that this will require me to start the chips up in a different mode and then reprogram the PPL in software on startup, is there any other considerations or problems with doing this??

Any input would be appreciated

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Postby bearcat » Thu Mar 03, 2011 7:12 pm

Be sure to read the XMOS doc:
XS1-L Link Performance and Design Guidelines.pdf

This document has several paragraphs on the physical link.

One question I have asked myself: Is it better to add series termination with some small resistors, like 33ohms, to a link line that is over a couple inches, or even 2 inches? I have implemented a 2 bit XLINK in a design with a straight wired link with about 2 inches of length, and works fine. The traces appeared to have quite a bit of ringing, but it is tough to probe such a high speed link properly, and could be artifacts due to the probe.

The spec recommends spacing the traces to help prevent crosstalk. With 10 lines that would seem to take quite a bit of PCB space. I also don't think you can route them without any crosses or via's.

The PLL setting should be able to be handled via the XN file.

The JTAG port I followed the XK-1 XMOS schematics. Need to chain the TDO and TDI ports as they show. I used a 33ohm on both TDO ports as the examples uses. I did not need the switch on TDO since my design will always have 2 L1's.

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