One vs two ttyACM interfaces

[mlac]

I use Linux with the nRF52840DK. I have three such developer boards, and only the one with the serial number 1050205291 exposes two ttyACM interfaces on Linux vs the other two boards with the serial numbers 683150769 and 68350417.

The processor of the 1050205291 board on which Segger runs is visibly smaller than the other boards, which might explain the difference.

In some situations, the two ttyACM interfaces are useful, so I'd like to use both on all my boards. For example, the "Bluetooth: Central UART" demo utilizes a ttyACM interface for logging and the other for communicating via UART.

Please let me know what causes the difference and how I can make all my boards expose two ttyACM interfaces.

[Hieu]

Hi mlac,

Regarding the two interfaces you observe with the rev 3.0.0 board, unfortunately that is just a known issue of the debugger firmware. Only one of them works. If you take a look at board's schematic, you will see that only one VCOM port is actually physically connected to the nRF52840 chip on the kit.

Therefore, while having two interfaces is handy for various use cases, it is not actually possible with the nRF52840 DK.

Which is why I am a bit surprised that you were able to use both ports for debugging the Bluetooth: Central UART sample. Are you sure you actually see both ports working as Serial/UART interfaces? The logging interface in that sample should be Segger RTT.

Hieu

[mlac]

Hi Hieu,

I can consistently reproduce this phenomenon. After flashing the Bluetooth: Central UART example, with this board:

/dev/ttyACM0 and /dev/ttyACM1 appears.

With this board:

only /dev/ttyACM0 appears.

Where can I buy more of the first type? :)

[Hieu]

Hi mlac,

Please read through this carefully and follow my recommendations before you make any purchase.

I really don't want to be the reason you make a purchase you later regret; or fail to stop you from making one.

mlac said:

Where can I buy more of the first type? :)

That would be nRF52840 DK rev 3.0.0, in contrast to your second kits which use a Segger hardware like the nRF52840 DK Rev 2, 1, or 0.x.x.
It seems the distributors linked to our Product Page's Buy Now don't often list what revision of the board they will sell you. If you wish to purchase a specific revision, you could fill out a Sales Related Question on our Contact Us page.

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BUT before you buy anything, I strongly recommend you to review your observation.
Prior to my previous answer, I have checked the nRF52840 Rev 3.0.0 Schematic , and checked the Bluetooth: Central UART sample carefully.
On the hardware, there is only one UART bus from the nRF52840 connected to the Interface MCU.
On the sample software, there is only one UART port configured for the purpose of talking to the BLE UART Service, and the log is produced via Segger RTT.

Your two ttyACM ports will always be there as long as you keep the board plugged in and powered on. However, only one of them would ever work.

If you don't trust me, perhaps you can trust another DevZone user with whom I discussed on the same topic recently?

[mlac]

Hi Hieu,

I've just tried to reproduce the issue, and the second ttyACM interface of the rev 3 kit seems non-functional, indeed.

Given the above, it doesn't make a difference whether I use the rev 2 or rev 3 kit, although I'll probably stick to my rev 2 kits because the extra non-functional ttyACM interface of the rev 3 kit slightly bothers me.

Thanks for everything!

[Hieu]

Hey, no problem! Everyone would agree with you that the 2nd non-functional port does more harm than good. 

On that note, I will just keep on pushing the back team to do something about that every time we have a ticket about it until it is solved.

I will close this question then