openthread coprocessor sample from sdk v3.2.1 doesn't work for me with openthread border router

RE: openthread coprocessor sample from sdk v3.2.1 doesn't work for me with openthread border router

Michael Hunold — Wed, 25 Feb 2026 15:24:43 GMT

I just want to let everybody know that I ran into exactly the same issue.

I wanted to rebuild my OpenThread border router setup using a Raspberry Pi 3B and a nRF52840 USB Dongle.

I went directly for the OpenThread coprocessor sample from v3.2.1, but this failed in the way described here.

I googled for the problem and came to this ticket. I switched to the OpenThread coprocessor sample from v3.1.0 and it worked out-of-the-box.

Regards!

RE: openthread coprocessor sample from sdk v3.2.1 doesn't work for me with openthread border router

Jan_Erik — Mon, 16 Feb 2026 17:47:25 GMT

I should have said that the target is the nrf52840dongle, not connected to a dk with a segger chip.

Removing all other usb devices had no effect.

Just to be clear: The setup works perfectly when I flash the dongle with the coprocessor bult with nrfconnect sdk v3.1.0. (and earlier versions.) It is only with v3.2.1. that the border router fails to connect with the coprocessor.

While troubleshooting this issue, I discovered that the v3.2.1 has apparently changed the vendor ID for the dongle coprocessor.
With v3.1.0 I get:

lsusb
Bus 001 Device 003: ID 1915:0000 Nordic Semiconductor ASA Thread Co-Processor

with v3.2.1, I get:

lsusb
Bus 001 Device 016: ID 2fe3:0004 NordicSemiconductor CDC ACM serial backend

Given the udev rules in the only Nordic Semi udev ruleset that I have been able to find (https://github.com/NordicSemiconductor/nrf-udev)

# 71-nrf.rules
ACTION!="add", SUBSYSTEM!="usb_device", GOTO="nrf_rules_end"

# Set /dev/bus/usb/*/* as read-write for all users (0666) for Nordic Semiconductor devices
SUBSYSTEM=="usb", ATTRS{idVendor}=="1915", MODE="0666"

# Flag USB CDC ACM devices, handled later in 99-mm-nrf-blacklist.rules
# Set USB CDC ACM devnodes as read-write for all users
KERNEL=="ttyACM[0-9]*", SUBSYSTEM=="tty", SUBSYSTEMS=="usb", ATTRS{idVendor}=="1915", MODE="0666", ENV{NRF_CDC_ACM}="1"

LABEL="nrf_rules_end"

I wouldn't expect the v3.2.1 coprocessor to work without changing the permissions to 0666.

Unfortunately, however, manually changing the permissions did not resolve the inablitity to communicate with the v3.2.1 coprocessor.

RE: openthread coprocessor sample from sdk v3.2.1 doesn't work for me with openthread border router

Priyanka — Mon, 16 Feb 2026 13:23:26 GMT

Hi Jan,

Could you try to unplug all other USB cables and plug in only the on that you intend to use for RCP?

Also, could you try to force the HW flow control ? To avoid race conditions etc.

So you could first disable MSD, i.e. the J-Link, then force the HW flow control and then perform a power cycle to apply this.

-Priyanka

RE: openthread coprocessor sample from sdk v3.2.1 doesn't work for me with openthread border router

Jan_Erik — Fri, 13 Feb 2026 16:52:55 GMT

It's similar in the sense that the error message indicates that the border router is unable to communicate with the rcp. It is different in that it is not due to a connection issue. As I said in the original post, I have installed the border router version that is recommended in the documentation for sdk v3.2.1. A coprocessor built with sdk v2.8.0 and v3.1.0 work with the border router but the coprocessor built with v3.2.1 does not.

Jan Erik

RE: openthread coprocessor sample from sdk v3.2.1 doesn't work for me with openthread border router

Priyanka — Thu, 12 Feb 2026 11:16:12 GMT

Hi Jan,

Could you take a look at the solution mentioned in this ticket for a similar error and let me know how it goes.

-Priyanka