Arty A7-35T // PMODb, PMODc Is the signal path of the connector taken into account?

[Henrik]

Hi,
I am currently considering connecting an "IF Recever" to an Arty A7-35T via the PMODb and PMODc connections. During my basic design considerations two questions have come to my mind:

1. If I understand the circuit correctly, then all signal lines at the two PMOD connectors are matched in terms of impedance and length. Do the signal lines of PMODb and PMODc also have the same length or are the lengths only the same for each connector?

2. Are the 200 mil length differences in the PMOD connector taken into account (difference between lower and upper row)?

I would be very grateful for a short answer!

[zygot]

The inappropriately named differential PMODs found on Digilent's FPGA boards match n/p pair lengths for each IO bank pin pair. Digilent does not match pin lengths across all pin pairs ( i.e all 8 signals ). Furthermore, the matching only extends to the connector through-hole pads. The right-angled connector used on all PMODs is unsuitable for most differential applications. If you need the best common-mode performance, then none of these PMODs are suitable. Also, since none of the PMODs have signals connected to IO banks with a Vccio that isn't 3.3V, TMDS_33 is the only possible differential standard. Lastly, placement of termination resistors can never be ideal.

If you need matched n/p pair lengths for an application where the FPGA pins are receivers you can always use IDELAY to compensate for mismatches, up to about a ns or so. In theory one could use a right-angle connector on an attached board that is in the opposite orientation to cancel out the connector length mismatch, but you still have to deal with the connector unsuitability for differential signalling.

There might some low frequency applications where these PMODs might be OK. like RS-422.

It would have been nice if Digilent actually put usable differential IO connectors on their boards that were connected to IO banks powered by a suitable Vccio; but they have refused to do so. I suspect that's because none of their PMOD accessory products use differential signalling or high speed signalling and the purpose of the PMODs is to sell PMOD products, not to make their boards suitable for custom user projects.

[Henrik]

Thanks for the quick and detailed reply! Since what I have in mind is close to the speed limits of the FPGA, I will probably refrain from trying this board... I would also be willing to swap the connectors on the board, but if the line lengths behind them are not uniform I can save myself the trouble!