Nexys A7-100T: PMOD maximum recommended frequency

[Carlo27]

Hi all,

I am interested in Nexys A7-100T FPGA board. In the datasheet I do not find the maximum recommended frequency for input and output signals, trough the PMOD pins. I need to handle signals in the order of 10 MHz. 

Does anyone know if the PMOD pins of Nexys A7-100T are capable to correctly handle this frequency? Which is the maximum recommended frequency?

Thank you in advance!

Regards,

Carlo

[zygot]

Digilent documentation is somewhat random on this point. For some of its FPGA boards, they specify the stand PMOD at a 10 MHz toggle rate in the board's Reference Manual. For other boards and ( as I remember ) the PMOD specification doesn't mention this. I think that this might be somewhat conservative but a 200 ohm series terminator placed at a random distance from a driver is not ideal.

Most of the Digilent FPGA boards have two standard PMOD headers and 2 "high speed" headers. The standard PMOD headers have 200 ohm series resistors as device protection, and that might be important. The "high speed differential" PMODs don't have series resistors. The _n and _p signal pairs are somewhat length matched but these don't support differential signaling due to the IO Bank Vccio being 3.3V. The only exception that I know of is the ATLYS, with 1 differential PMOD with pins connected to an IO bank that can have its Vccio user selected. I've used these at 30-40+ MHz toggling rates for a couple of Digilent's boards in a few projects successfully. I don't believe that the _n/_p pair PCB traces are laid out is a differential manner, which is good because they can be used for signal-ended applications... which is the only differential IOSTANDARD available ( other than TMDS ) for series7 devices.

So, for most boards you get 8 slow GPIO and 8 faster GPIO, which isn't a lot unless you are going to use PMOD add-on boards... which is what the standard PMODs are designed for. I'm unaware of any PMOD add-on boards designed to be used with the so called "high speed" PMOD connectors on the FPGA boards.

Before selecting an FPGA platform do read the user reference manual and schematic. If these aren't provided then you might want to keep looking. Edited May 11, 2022 by zygot

[Carlo27]

Hi Zygot, 

thanks for your reply. I saw from the schematics that most of the FPGAs have not the protection resistances on the PMOD differential pins, so they should better for high speed signals.

However, in my project, I have to use the output pins as single-ended, is it possible and "safe" to do this with differential PMODs? In your past projects did you ever use them as single-ended? Did you see unwanted cross-talk effects?

Regards,

Carlo

[zygot]

Your concern about cross-talk between _n and _p pair traces is something to consider. Yes, I've been able to use the "high speed" differential PMOD connectors as single-ended signals for board to board connections on the Nexys Video, Genesys, and Genesys2 boards without noticing evidence of cross-talk. That's why I'm making the assumption that these signals, while matched to 10 mils or so, aren't laid out on the PCB as true differential signals. Also, I don't see any such traces on the top and bottom PCB layers; but this doesn't mean much as differential pairs can exist on separate PCB layers.

The 12-pin connectors are not particularly suited for high speed, as in a typical LVDS display application, but I've seen similar connectors, using true differential signalling, run at 500+ MHz on boards from other FPGA board vendors.

I guess that it all depends on what you mean by high speed, how many signals you need to accommodate, and the timing nature of your signals. There are a lot of details to get right for any specific signalling application. I haven't tried sending a couple of really narrow strobes through the Digilent "high speed differential" PMODs. I did once, a long time ago, make some custom PCB add-on boards that attempted to attach true single-ended to LVDS drivers and receivers to these PMODs and try and see how fast I could communicate bstween 2 FPGA boards. As I remember I got stuck and never got back to it, but the problem could have due to an error in the official Digilent master constraints files that I was using at the time. Always double check your routed pin location assignments against the schematic. Edited May 12, 2022 by zygot

[Carlo27]

Hi Zygot,

thank you very much for your support!

In my project I need 20 I/O single-ended signals (in particular I need 12 inout, 5 outputs and 1 inputs plus an input and an output clock) in the order of 8 MHz, a USB-UART and an ethernet port.

Do you have (or any other user has) an FPGA board to suggest?

[zygot]

Digilent sells its CMOD boards though the single ground pin is a problem. ( Well there are also 2 ground pins on the PMOD but you can't connect these directly to a custom PCB... which is really what the CMOD for factor is supposed to address ).

Personally, my cheap go-to FPGA board is the Terasic DE0 Nano. I have a few ot them dedicated as components in custom instruments. They are great for insertion into a custom PCB with 2 40-pin GPIO headers having sufficient ground pins, GPIO, and clock capable inputs. You can use the free version of Quartus with them ( this includes a fre version of ModelSim ). There's no PC connectivity, but I always have a couple of 3.3V compatible TTL USB UART cables and breakout boards available. The Cyclone 10 development board might be a good fits as it has everything that you need. The Terasic Cyclone V GX Starter Kit is a nice low cost platform ( though no Ethernet unless you buy an add-on HSMC board. The MAX10 Development kit is is nice but there are no MAX10 devices or boards available anywhere that I know of. BTW, no one reqards me for mentioning a vendor product... I'm just thinking of what I use and have found to be useful. For Intel stick with Cyclone IV, Cyclone V, MAX10 ( well perhaps in the near future ) or Cyclone 10LP if you are working on a budget. I really wich that some Xilinx FPGA board vendor would come up with a Artix version of the DE0 Nano.

In the Xilinx world the Mimas A7 is reasonably priced and feature rich with plenty of IO, including a 1 GbE port. It's pretty hard to find cheap Xilinx based boards with sufficient IO for use as a general purpose project these days.

Unfortunately, this isa bad time to buy any electronic components. Everything that is available in distribution is more expensive than a year ago... and there isn't a lot in stock.

Sometimes you have to be resourceful. A really cheap board like the DE0 Nano can have one of its GPIO headers connected to a Raspberry Pi 3 or 4 and the other connected to you project external hardware. You can get a good 3 MB/s out of the SPI ports with an SCLK of 31.25 MHz and 12-40 MB out ot the ALT1 pin functions (SMI); at least for small blocks of data. The RPi makes the Ethernet part pretty straight-forward ( and if you want to have full Etherent capability this is not the case for any FPGA platform ). You might garner snickers with such a setup but if it gets the job done quickly and relatively painlessly those are easy to ignore. If you want to use the RPi as an add-on board for your cheap FPGA board make sure to use the latest OS release, if for no other reason than the ability to use ramdisk(s) for temporary storage Edited May 12, 2022 by zygot