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DPA

High speed output on PMOD ports

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Hi,

I trying to select an FPGA development board that will allow high speeds (100MHz+) signals on the output connectors.  Ideally on the PMOD connectors.

Can this be done with the Nexys 4 Artix 7 board?  If not can you recommend a development board.

Thanks.

D.

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Hi @DPA,

We would not be able to guarantee 100 MHz from the Pmod I/O Ports on the Nexys 4 artix 7 board. We do not have a metric for max frequency on the Pmod I/O Ports. Here is a forum thread that discusses this. The  Zedboard has two of the Pmods, JC1 and JD1, that are aligned in a dual configuration and have their I/O routed differentially to support LVDS running at 525Mbs as discusses in the reference manual here. The FMC's on the Zedboard, Nexys Video and the Genesys 2 will be able to handle 100 MHZ.

cheers,

Jon

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2 hours ago, DPA said:

I trying to select an FPGA development board that will allow high speeds (100MHz+) signals on the output connectors

So an adequate answer to your question is a bit complicated.

Do you want outputs or inputs?

How many outs and ins do you need?

Single-ended or differential?

What logic standard are you connecting to? The answer to this question is leads to the next one which is what is the termination scheme?

Most Digilent PMODs have, for good reasons appropriate to their typical user need, series resistors. This is usually not consistent with ideal transmission line design.

If you have the tools to make an FMC mezzanine board interface then that's a possibility. I've done projects with these connectors in my DYI PCB oven. The PCB layout and component footprint wan't easy... the results were good enough for the job at hand but not merchantable.

I'd look at small FPGA modules with high density connectors and good support.... I don't have experience with any to make specific suggestions. Look at Opal Kelly, They have a new Artix board with their new SYZYGY ports and a USB 3.0 interface. If you can deal with their niche philosophy this might be good. I have used them in the past. Find my latest post about it in the Technical Based Off-Topic section for more commentary.

There's no way to give you a good answer without knowing what you want to do.... 

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Hi All,

Thanks for your responses here.

Fundamentally what I am trying to do is create a simple open loop PWM output that can run as fast as I can get it for a test system.  I'd like 100MHz on the output.

I'll read the detail on @zygots Differential PMOD Challenge.  

I'm a novice to FPGA, more an analog guy.  I have a Nexys 3 Spartan 6 but haven't tried it yet.  However, given that the Nexys 3 is no longer available I wanted to start with a new board.

To answer zygot's questions: 

One to four single ended outputs.  PWM data will be set with simple on board switches so no high freq input requirements.  The outputs will be connected to a test board whose inputs will be CMOS.  Ideally I'd like to get my hands on the simplest FPGA IC that I could design into my test board but don't know if such a device exists.

I guess for the moment my starting point will be to pass a simple signal through the Nexys 3 I have and see how fast I can get it go.  I'll keep you updated.  

Thanks again.

 

 

 

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7 hours ago, DPA said:

One to four single ended outputs

In this case you may well be able to use a differential PMOD. Just assign the signals to the _p or _n pins of each differential pair and assign the corresponding opposite polarity pins to GND. You should have no trouble with 100 MHz outputs as long as you pay attention to details in terms of getting the signals to your receiving equipment. You can determine which are which by the FPGA pin names on the schematic. You probably want some form of termination at the receiving end to minimize reflections. In the constraints file you might want to use the lowest appropriate drive strength and slowest appropriate slew rates on the driver. If you cable all 8 PMOD signal pins ( 4 signal and 4 GND ) plus the 2 dedicated GND pins you might be OK.

At around 100 MHz and above digital is analog for all practical purposes so this is right up your alley.

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23 hours ago, jpeyron said:

Hi @DPA,

We would not be able to guarantee 100 MHz from the Pmod I/O Ports on the Nexys 4 artix 7 board. We do not have a metric for max frequency on the Pmod I/O Ports. Here is a forum thread that discusses this. The  Zedboard has two of the Pmods, JC1 and JD1, that are aligned in a dual configuration and have their I/O routed differentially to support LVDS running at 525Mbs as discusses in the reference manual here. The FMC's on the Zedboard, Nexys Video and the Genesys 2 will be able to handle 100 MHZ.

cheers,

Jon

So an adequate answer to your question is a bit complicated.

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I hate myself for having to defend any use of those silly differential PMODs but I'll make the following observations:

  • The FMC is a very complicated high density connector designed to connect to a well designed mezzanine card thst's plugged into it. It's NOT a general purpose GPIO connector suitable for breadboarding a signal source to external devices.
  • You likely don't want or need the complexity of doing a ZYNQ based design just to put out a couple of PWMs. I use and like the Zedboard but it's not what you want.
  • There are much cheaper solutions available from Digilent with those (usually $%^!) differential PMODs. It's unfortunate that Digilent can't be bothered to create pdf versions of it's User's Manuals any more so you have to do online comparison of boards. I looked at the Arty A7 board and they don't mention the usual toggling rate verbiage you will find for the description of the high speed PMODs that user's manuals for other boards have... but I doub't that the performance is all that different for your needs.

You've done something that I've been unable to do, which is find a use for one of these things.

Oh, and no your board doesn't have a high speed PMOD.

Edited by zygot

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