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pmjobin

Unstable USB connection with Cmod A7-35T

Question

Hello,

I recently purchased a Cmod A7-35T kit and its USB connection is so unstable that it renders the device almost unusable. I've tried every combination of 2 different host machines (including a laptop running on battery power to rule out a noisy power outlet as a potential cause) and 4 different micro-USB cables but to no avail. I've had somewhat more luck with the shortest USB cable as I was finally able to upload a design and test it after fighting against intermittent disconnections for a few hours.

While using the hardware manager in Vivado, I can see the A7-35T appear and disappear about every 5-10 secs. Every time it disappears, the following error messages are logged:

image.png.ceee6e8f21edb9668e4e02b450181f63.png

But these connectivity issues aren't limited to Vivado. I can see the same thing happening in the dmesg log while Vivado is not even running.

I've browsed the forum a bit before posting this and noticed that a few people have experienced issues similar to mine with the Cmod A7. It has been suggested that using higher quality USB cables can prevent intermittent disconnections but I'd expect at least one of the 4 cables I tested it with to be of adequate quality. I've used the same cables with other dev kits in the past and never experienced such problems.

I was wondering if a definitive fix has been identified (such as a specific USB cable you can recommend me) or if my board is simply defective.

Thank you,
PM

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Hi @pmjobin,

You can find a list of manufacturers that have certified Micro-B connectivity on the USB.org site here, however you'll note that the vast majority of the cables listed are type C to micro B, rather than type A to micro B, so be aware that your options will be somewhat limited (and if it matters to you) not necessarily readily available in whatever country you may be in. After some searching, I was able to find this certified USB 2.0 A to micro B cable from a Swedish based company (link), but none of us here at Digilent have tried this specific cable. I wasn't able to readily locate an Evernew USB cable that was available for purchase.

Otherwise, these two other cables from Amazon (link1, link2) may work, but note that they are not USB-IF certified (nor have those of us at Digilent tried these specific cables.

Thanks,
JColvin

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Hi @pmjobin,

One cable that we would suggest is an Evernew cable making sure that it is a USB .org certified high speed USB cable.  Here is a forum thread that discusses this issue in detail.

thank you,

Jon

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Hi @jpeyron,

I had seen the thread about the Evernew USB cable before posting but I've been unable to find anything about it online. Would you mind providing me with a link where I can purchase this cable (or any USB.org certified cable that is known to work with the Cmod-A7)?

Thank you,

PM

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Hi @JColvin,

Thanks for your help. I would never have expected that all four cables I had at my disposal did not respect the USB 2.0 standard which is so ubiquitous, but after further research, it appears that a worrying number of micro USB cables sold on the consumer market are so-called "charging" cables and are unable to operate at high speed USB. Obviously, I had to learn this the hard way. I purchased the AmazonBasics cable you suggested and I can confirm that these are suitable. I suppose that most cables explicitly rated at 480 Mbps would work equally well.

May I suggest that you add a warning against micro USB cables advertised as "charging cables" (or lacking USB 2.0) on the Cmod product page or reference manual to prevent other users from falling into the same trap?

Thanks again and best regards,
PM

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Hi @pmjobin,

Yes, we can make a note to the Cmod page (and other pages as appropriate) to be wary of charging only cables.

Thank you for the feedback.

Thanks,
JColvin

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We have found that a 100nF capacitor attached across VBUS and ground near the connector makes the Cmod A7 more tolerant of out of spec cables and we will consider adding it to a future revision.

There aren’t any pads for the capacitor, which makes it a bit challenging to solder in a 10V, 100nF ceramic capacitor. We managed to stack it on top of half of D1 and half of C65, which are both on the bottom of the board.

image.png

When testing with USB.org certified cables, we haven't been able to reproduce any connectivity issues.

We also recommend plugging in the cable into a port directly on the motherboard or into a USB hub instead of a front panel USB port.

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