Nexys Video Board

[jlab_xilinx]

Hello,

I am working with the FMC on the digilent video board. Most of the pins appear to be mapped correctly but I am reasonably confident that some of the FMC pins do not match the schematic.

 

Has anyone else had issues with the FMC pinout?

Is there a way to get a hold of the actual board files to verify the FPGA to FMC trace routing agree with the published schematic?

Thank you,

James

[zygot]

I've used the Nexys Video LPC FMC connector for quite a number of projects using a few different mezzanine boards without issues. I've always taken the time to trace through pin connectivity.

While Digilent has certainly messed up the master constraints and documentation for a number of its boards the schematic ( except for the NetFPGA-1G-CML board ) has always been reliable, in my experience. Generally, schematics are from the PCB layout tools so it would hard to make mistakes.

Which pins are concerning you and why do you believe that the boards are different than the schematic ( assuming that the schematic rev level matches your hardware )? Edited April 26, 2022 by zygot

[jlab_xilinx]

Hi Zygot, thank you for your reply.

 

Where should I look on my board to make sure the schematic matches the actual board rev?

 

Thank you for your response. The pin in question is FPGA pin L18 which goes to FMC_LA02_n.

I am using this L18 pin as a reset pin on a mezzanine board that connects to the FMC. Probing it with an oscilloscope, I don't see it resetting. I have other similar pins which I do see resetting but not L18. I also have tied this pin to a constant HI (and some other pins just as a sanity check). I see the other pins as HI's but not L18.

I also tried to use a weak pull up to force the pin HI (while driving L18 pin HI, as a constant), but the FPGA appears to be pulling the pin low.

I also tried ohming out this connection to ground and I don't see any shorts to ground for pin L18.

 

Do you have any additional debug suggestions?

 

Thank you,
James

[jlab_xilinx]

Also, underneath the Digilent board I see the rev number. It is REV A and I have been using schematic with REV A.0 (which I assume is the same?)

[zygot]

The pins on the FMC connectors can get bent, but I assume that you've already done an inspection on both the carrier and mezzanine boards for connector mechanical issues. I've used that pin on my board which is a Rev A version, so this isn't something that I've seen. Digilent isn't very good at providing explicit board revision change notices so I don't know what's different about REV A.0; but I'm assuming that is due to a component change due to availability.

It's possible that there is a fault on your particular board, though I assume that Digilent has every board checked for connectivity. It's possible, but unlikely, that a gouge or mechanical stress has  broken a PCB trace. It's possible something conductive has gotten under the FMC connector and is causing a short. It's also possible that the FPGA has had that particular IO abused and can't source or sink current. Those are things that spring to mind that I've come across testing out custom boards that I've designed and built.

That's all I have at the moment.

Edited April 26, 2022 by zygot

[jlab_xilinx]

Thank you for your feedback.

If I probe FMC pin H8 (which is FPGA pin L18) I see a near short to ground (several ohms).

My best guess is that one of the ground pins on the FMC has a little solder bridge to FMC pin H8.  I am doing this measurements with the FPGA off and unpowered (and not plugged in).

 

Maybe this is a little risky, but what if I apply a 3v high amperage voltage to this pin and the nearest FMC ground pin. The goal would be to burn the solder bridge away.

What do you think?

 

-James

[Richm]

I think that might be a good way to permanently "fix" it. If you are looking for a solution that involves tossing out the board you have and buying another, this is it.

 

[jlab_xilinx]

Hi Richm,

Thank you for your response. What if I configure that specific FPGA pin to be HI-Z output, what is the maximum current that I can supply to the path between that pin and ground in order to 'burn' off the solder bridge? My plan is to make a dummy .bit file which sets all of the pins to hi Z output (3.3v). Then, I would inject a 3.3v DC voltage at a low current between the board's ground and the FMC pin in question.

 

My thought it to apply only what the pin is rated for (10 mA, 100 mA maybe?) when configured in HI Z. I would very much like to not destroy this board.

 

Thanks again,

James

[elodg]

All expansion connectors are tested in manufacturing for short to ground, short to neighbor, and then continuity with a loopback board.

What you are experiencing sounds more like an ESD damage to the FPGA pin. If you were to drive current through there, you might produce more damage.

[zygot]

ESD is the most common way to abuse electronic devices, other than possibly trying to drive a pin that's shorted to ground or some voltage source. Either way, it's unlikely that only one pin has been affected. That doesn't mean that your board is unusable... just that you will not have certainty that your designs will work the way that they might on a healthy board.

Another way to abuse an FPGA board is to connect an FMC mezzanine card to a carrier and try to use it without first mechanically securing the boards together, or worse yet trying to plug/unplug the mezzanine card while the carrier is powered. Edited April 28, 2022 by zygot

[jlab_xilinx]

Thank you all for your feedback.

 

I assumed Digilent had good quality control for their products, it must have been damaged at some point in our Lab. We will just need to find a way to work around this one pin.

 

Thanks,

James