[Cryptech Tech] Comments on Alpha Board schematics

[Joachim Strömbergson]

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Aloha!

Pavel Shatov wrote:
> Can one use RGMII instead of GMII? We currently have two 8-pin GPIO 
> headers for FPGA, this is enough for RGMII, which is only 12 pins. 
> Speaking of speed, Artix-7 I/O is fast enough, the only tricky thing
> is that receive clock should be routed to MRCC pin (independently of 
> whether we use GMII or RGMII).

I see no big problems with that. The RGMII interface is a bit more
complex since data is clocked on both flanks to get the needed data
rate. But the requirements on routing would be the same (parallel data
to/from pins @ 125 MHz). If we want to be compatible with RGMII 2.0, the
I/Os used in the FPGA must support HSTL.

Looking at the 7Series Selecti I/O Guide this means that we need to use
HP bank type for the GPIOs. And we would only support single ended HSTL.

http://www.xilinx.com/support/documentation/user_guides/ug471_7Series_SelectIO.pdf


So, unless we want to add even more headers on the board for the FPGA
(dedicated for a possible high speed interface), being able to use
existing headers is a good solution. But we should probably change from
two 8-bit headers to one 16-bit header (or a single 2x8 dual row header)
too more easily keep the traces aligned.

In short: Changing to a single header with 16 pins, noting requirements
on routing to support signalling @ 125 MHz and connecting to I/Os in the
FPGA that supports HSTL should be enough at this stage to open up for a
high speed interface in the future. I think that is a reasonable amount
of feature creep.

- -- 
Med vänlig hälsning, Yours

Joachim Strömbergson - Alltid i harmonisk svängning.
========================================================================
 Joachim Strömbergson          Secworks AB          joachim at secworks.se
========================================================================
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