Arty-Z7 High Speed PMOD Differential Signals

[DanielHP]

Hello there,

I'm planning to use the PMOD high-speed connector from the Arty-Z7 as I need to drive signals around 23MHz. So the recommended way to do that (>10MHz) is by using differential signals.

I see from the reference manual that the 100 ohm trace impedance and 0 ohm shunt resistors are ideal to achieve that requirement. However, I noticed the FPGA pins are configured to work with 3.3V signals only.

I plan to use a LVDS receiver on the other end with it's corresponding 100 ohm. How can I pass these signals to match the LVDS requirement? Otherwise, is there another differential standard that I can use with 3.3V signals?

Thank you,

Daniel.

[zygot]

The only differential IOSTANDARD that is supported for Series 7 devices is TMDS_33. TMDS_33 is intended for specific applications, mostly video.

There are a lot of discrete LVDS standards and ICs out there so you have to figure out how to meet all of the dc specifications for the LVDS receiver that you want to use. These devices cover a range of logic voltages. Maxim-ic ( ADI ) has some nice LVDS <-->Single-ended interface devices that are easy to use ( some with internal termination for the receivers )

While you can create plenty of issues driving a signal through an ill-conditioned transmission line i wouldn't call 23 MHz high speed. The best path might be to use a single-ended to LVDS driver IC between your LVDS sink and one pin of the FPGA. You can eliminate many issues by using the lowest drive current possible and reduced slew rate constraints for your FPGA outputs.

Digilent PMOD headers are not designed for custom applications. Digilent so called high speed differential PMOD header do not support differential signalling. I haven't seen any indication that this is going to change, or that Digilent customers will stop being confused as to the suitability of their FPGA boards for a particular purpose.

You've probably seen this recent thread: https://forum.digilent.com/topic/26376-gensys2-diff-clock-route-to-pmod/

One out of many over the years.

There are cheap FPGA boards that support differential signalling if that's what you need.

Edited July 28, 2023 by zygot

[zygot]

49 minutes ago, DanielHP said:

Yes, it would be better to explain this detail beforehand in the board's reference guide. It is a consuming task just to realize we cannot use the differential line as expected.

If this was an isolated thing, that would be one thing. It isn't and I've been trying to get Digilent to acknowledge this for about a decade now. They've shown no signs of changing their ways, as the thread that I provided above illustrates. I don't get it... I'd expect a company as big as National Instruments to care about their reputation, and their customers' time.

 

[DanielHP]

Hi zygot, I appreciate your time helping me figure this out.

We still want to take advantage of this board's differential-capable pins, but as you mentioned, adding an LVDS receiver in between to leave these chips (driver/receiver) to handle the actual differential signaling.

Yes, it would be better to explain this detail beforehand in the board's reference guide. It is a consuming task just to realize we cannot use the differential line as expected.

Best,

Daniel.