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kami

Nexys Video Pmod Connectors

Question

Hi,

I'm planning to implement a video processing design on Nexys Video Board. I need to capture frame_valid, line_valid, data_valid, pixel_clock(40MHz) and 16-bit parallel pixel data from a camera in LVCMOS33 voltage level. I got a few questions about Pmod connectors.

1) In reference manual of Nexys Video Board, it says JA Pmod connector is routed to single ended FPGA pins and it supports <10MHz and LVCMOS33. Is there any problem If I use this port for 40MHz inputs such as pixel_clock, frame_valid or data_valid? (I checked the pin-outs and JA connector pins are not connected to clock capable pins of FPGA.)

2) Reference manual states that JB and JC pins are connected to FPGA differential pair pins and they support >10MHz LVDS25 or TMDS33. Can I use these differential pairs as 2 discrete single ended LVCMOS33 @40MHz inputs? (I'm planning to capture 16-bit parallel video data from these connectors.)

Best regards,

Kami

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Posted (edited)

@kami

Do you think that Digilent would say that their low speed PMODs support < 10 MHz signal rates if they believed that it was 40 MHz?

As to the differential PMODs you are incorrect. The reference manual states that they support LVDS_25 inputs or 3.3V IOSTANDARD signalling. All Digilent differential PMODs pins are assigned to 3.3V IO banks. Possibly you can use them for LVDS_25 inputs. You can certainly use them for TMDS_33 IO [ though you will have to supply the termination and it will be at a less than optimal location ]. The reference manual states that these PMODs are routed as differential pairs so using them as single-ended IO is problematic during switching. You can use the 4 _p or _n pins for signalling and assign the others as GND or perhaps static control signals. It's apparent that Digilent throws left-over IO pins into the PMODs as it's rare to find one with a clock capable pin, though it does sometimes happen.

The Nexys Video has a very capable LPC FMC connector suitable for what you want to do.

It's shameful that Digilent continues to confuse customers with the differential PMOD farce when it would be so easy to provide one really useful IO port. Questions similar to yours comes up often. Fun quick quiz. Over the many years that Digilent has been making FPGA boards with differential PMODs how many compatible 4 differential pair PMOD add-on boards has Digilent designed and supported for any of those FPGA boards?

In general picking an FPGA board and then trying to make it do something that it wasn't designed to do is a bad idea. Things get even more complicated when trying to use add on boards not supported by the vendor.

Edited by zygot

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Hi @zygot,

Thanks for your reply.

About the frequency issue, I just wanted to know if anyone tried >10MHz on single ended Pmods.

I haven't choose a board for implementation yet. Nexys Video looks promising but it looks pmods are not suitable for me, as you said. I think Xilinx FMC XM105 Debug Card will be suitable for the inputs that I mentioned.

By the way, differential pmods can be useful for low cost, low FPS parallel output cameras. I think there is a camera that works with pmod connectors, but i don't know if there are more add-on boards exist.

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19 minutes ago, kami said:

I think Xilinx FMC XM105 Debug Card will be suitable for the inputs that I mentioned

Perhaps. I use this board from time to time. Be aware that all of the XM105 FMC signals are routed as single-ended, which is what you want, but I believe [ check on that ] that the Nexys Video FMC signals are routed as differential pairs ( which is closer to the Vita57 standard ). At least you have a lot of signals to work with to mitigate differential coupling if it's a problem. With a wide bus ( I'd consider 16 bits even for 40 MHz as wide ) there's the probability that some reasonable trace length matching will be necessary.

26 minutes ago, kami said:

I think there is a camera that works with pmod connectors

I've heard of people who like using snow shovels as a sled to race down hills (Not interested thank you). If you buy an FPGA board as a platform to develop your own stuff it should provide IO interfaces that are designed to be used in a technically correct manner. No one wants to spent a lot of time and money seeing if they can get lucky enough to get by with a deficient interface. Even if Digilent routed the signals to a connector with decent signal integrity characteristics as 8 high-speed singe-ended lines that would be a smallĀ  improvement over what they offer with the differential PMODs.... there are so many ways to improve and no way to make them worse.

Digilent does so much right; it's a shame that they've strayed from their past and taken the much more capable Series7 device boards to where the casual user has to be restricted to either 8 signal PMODs that are designed for <10 MHz or, in the case of the Nexys Video and Genesys2, a high pin count SMT connector that most user's won't have the means to build a custom board to utilize. I'm mystified. I get frustrated reading all the posts of people who want to do neat projects hoping that their board will let them do it.

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