Zephyr PPP modem times out on CMUX=2 on nrf-sdk 3.1.0

RE: Zephyr PPP modem times out on CMUX=2 on nrf-sdk 3.1.0

Benjamin — Mon, 18 Aug 2025 10:34:07 GMT

This issue has been fixed in https://github.com/nrfconnect/sdk-nrf/pull/23973. It did not make 3.1.0.

RE: Zephyr PPP modem times out on CMUX=2 on nrf-sdk 3.1.0

Konstantin Klitenik — Fri, 15 Aug 2025 14:08:50 GMT

Modem FW: mfw_nrf91x1_2.0.1. (SDK 3.1.0)

nRF54 FW: custom zephyr application (nrf-sdk 3.1.0) with zephyr's cellular modem driver via PPP.

RE: Zephyr PPP modem times out on CMUX=2 on nrf-sdk 3.1.0

Benjamin — Fri, 15 Aug 2025 11:34:38 GMT

Hi Konstantin,

It seems like you found a solution. I will forward this to the developers. What MFW version do you use and what FW are you running on the nRF54?

Best regards,
Benjamin

RE: Zephyr PPP modem times out on CMUX=2 on nrf-sdk 3.1.0

Konstantin Klitenik — Wed, 13 Aug 2025 19:16:04 GMT

Based on some more debugging, if I change this line in the modem_cellular.c driver

MODEM_CHAT_SCRIPT_CMD_RESP("AT#XCMUX=2", ok_match)

MODEM_CHAT_SCRIPT_CMD_RESP_NONE("AT#XCMUX=2", 200)

it seems to work. So it almost seems like the previous version (sdk 3.0.1) was sending OK on the original channel, while the new SDK queues the OK and then immediately updates the DLCI channel. When that OK goes out, it goes out on the new channel, while the zephyr driver is still waiting for OK on the initial channel.