PMOD max data rate in Basys3 board

[davidhergueta]

I have a question regarding to the Basys3 board of Digilent. Which is the maximum data rate that can transmit the PMOD ports of the board? I don't see this information in the reference.

I would like to drive signals at at least 100Msps more or less

Thank you in advance

Edited January 10 by davidhergueta

[artvvb]

Hi @davidhergueta

Welcome to the forum.

The max rate isn't specified. You can find other users trying to get Pmods going as fast as possible in various places around this forum, going back quite a while: 

We've comfortably been able to push 10+ MHz serial clocks for SPI interfaces through Pmod ports, but I would not expect 100 MHz to work.

Thanks,

Arthur

[zygot]

"The max rate isn't specified. You can find other users trying to get Pmods going as fast as possible"
It's hard to argue with a statement that doesn't make any assertions.

Be aware that all of the PMODs on the Basys3 are the "low speed" variety; that is they have 200 ohm series resistors between the FPGA pins and the PMOD connector pins.

If all you want to do is toggle pins at 100 MHz, yes you can do that. if you want to use such a signal to transmit information, then there's a lot more to consider.

Only the so called high speed differential PMODs have any PCB trace length matching. That's only between the _n/_p pin pairs. And no PMOD, except on n the ATLYS can do differential signalling.

None of the PMODs that I know of have length matching across all 8 pins.

Most PMODs don;t have a clock capable pin connected to any of the PMOD pins. This might be a problem for high speed interfaces.

10 MHz toggle rate is what most of Digilent's Reference Manuals suggest for the standard low speed PMOD. That's probably very conservative. I'd certainly implemented SPI interfaces using PMOD connector to external devices via a custom PCB adapter that exceed 32 MHz; on the high speed PMODs. I'm guessing from past experimentation that ~50 MHz is practical limit for useful educational work. Of course the termination on the receiving end, the quality of the transmission line from FPGA pin to receiver pin, the current drive, slew rate, etc will determine the quality of your signal. Large amounts of overshoot or undershoot will degrade performance and potentially reliability.

Basically, what I'm trying to say is that if you want a good answer to your question, then you need to ask a better question. A nebulous question invites a nebulous answer.

I suppose that what you really want to know is whether or not your design idea will work with a Basys3 PMOD connected to some external circuit. A good answer requires more information about what you are trying to do.

I looked over the current Basys3 Reference Manual and was surprised to see that it didn't mention a useful toggle rate for the PMOD connectors. Since that board is pretty old I assume that this information was scrubbed from the original manual. This seems to be consistent with the new Digilent policy of removing important information from easy access when it doesn't reflect well on the product capabilities and replace specifications with ill-defined comments that suggest something more positive. Edited January 10 by zygot