Connect two Arty boards via a differential serial interface

[Jammin]

I’m planning to connect two Arty boards via a differential serial (SPI-like) interface to evaluate high-speed differential communication between the two FPGAs.

I thought about using the high-speed pmod connectors and the TMDS_33 IOSTANDARD as suggested in this https://forum.digilentinc.com/topic/1728-arty-selectable-25v-io-banks-for-lvds/ discussion.

Is this feasible?

As far as I understand, I have to develop a pmod connection board with 50 Ω pull-up resistors on the TMDS_33 inputs, correct?

And (last question) is it possible to introduce a capacitive coupling between the 3.3 V outputs of the pmod headers as they are already connected via the 50 Ω pull-ups?

[JColvin]

Hi Jammin,

I have asked some of our applications engineers about the requirements needed to connect the two FPGA boards together; they'll get back to you here on the Forum.

Thanks,
JColvin

[jpeyron]

Hi Jammin,

Using the high-speed pmod connectors and the TMDS_33 IOSTANDARD as suggested in this https://forum.digilentinc.com/topic/1728-arty-selectable-25v-io-banks-for-lvds/ discussion is feasible.  You will have to develop a pmod connection board with 50 Ω pull-up resistors on the TMDS_33 inputs.  Could you clarify your last question to ensure that i'm unstanding it correctly.

thank you,

Jon

[Jammin]

Hi James, hi Jon,

Many thanks for your assistance!

I’ll try to clarify my last question:

I’m not sure what to do with the 3.3 V pins of the pmod headers. I think connecting the 3.3 V pins of the two Arty boards is not possible, as they are connected to two independent voltage regulators. That is why I thought about introducing some sort of coupling between the 3.3 V lines of the two Arty boards.

Is such a connection necessary? Or is it possible to use the 3.3 V pins of each board just for connecting the 50 Ω pull-up resistors to the inputs of that specific board, without introducing any connection between the 3.3 V lines of the two boards at all?

Thanks again for your answers,

Jamin

[sbobrowicz]

good point, you don't want to connect the two 3.3 V rails together. This could cause problems, especially when one Arty is turned on and the other is not (causing backpowering). 

I'm not an EE so this isn't my area of expertise, but I think you want to implement the CML AC coupling circuit shown in section 4.2.2. of this document for each of the TMDS pairs:

http://www.ti.com/lit/an/slla120/slla120.pdf

Either side of the coupling capacitor should be pulled to the 3.3V rail of the associated Arty board. That circuit should prevent back-powering issues too.