Cmod S7 and USB cable issue

[lfigiel]

I am interested in buying Spartan-7 FPGA Module:

https://store.digilentinc.com/cmod-s7-breadboardable-spartan-7-fpga-module/

but first I would like to know if it suffers same USB cable issue as CMOD-A7:

Since FTDI circuitries look similar on these two boards I just want to make sure the faulty design was not copied over from A7 onto S7.

[jpeyron]

Hi @lfigiel,

We reached out to one of our design engineers about this thread and they responded "We added an extra 100nF capacitor near the USB connector and that does appear to improve reliability when using a poorly constructed USB cable."

thank you,

Jon

[zygot]

@lfigiel,

I don't own the board that you are asking about, so I can't offer a direct answer. I do feel comfortable in making the assumption that whatever ails the A7 version of this series is not likely to be found by looking at the schematics. The discussion you refer to lays out a number of likely culprits. It also could be that a particular FTDI part (package) requires greater care to use. While the Spartan7 is a different animal than the Artix7 the modules are similar in size, component density and likely PCB stackup. I do not want to make any suggestions about whether or not anyone should or should not take a chance on purchasing one of these boards. Frankly, its a question that only a purchaser can answer.

I can say that I've used a number of the Terasic DE0-NANO boards with success. Those boards come with the Intel baggage that didn't exist when I purchased them and when Altera was an independent company. The price is about the same. My personal opinion about very small boards and FPGA devices is that they make for an unnatural couple. That's not to say that you can't do useful things with such modules. If you're going for a really cheap price point with such products then you are likely to have more potential issues because addressing them by design incurs production costs.  Note that the S7 board like its A7 cousins are shipping in an unusable state; that is without a USB cable**.

You ask a very valid question. There are other vendors of small FPGA modules at higher price points. It is up to the buyer to determine the quality of the operational specifications of any product, the openness and support of the vendor and intended use to determine if a purchase is prudent. This is not an easy task, even for very sophisticated customers.

** Here's an interesting story. Many years ago when I lived in the big city I had a neighbour offer, for  a price,  to cut down a very large tree branch on my property that was in danger of breaking and also happened to be hanging over another neighbour's back porch. Everything seemed to be going well until we got to the point of starting to sever the dangerous limb. My 'helpful' neighbour had tied a rope to the end of the limb and handed the other end to me and said "when you hear the branch starting to break pull hard' . Now, I've never been in such a position but it was very clear to me that anything that I might do with that rope wouldn't be noticed by a half-ton piece of wood set free to fly like a bird... just a thought that came to me as I wrote this. 

[vicentiu]

We don't have any reports of USB connectivity issues on the Cmod S7.