Using mikroe "LTE IoT Click" aka u-blox SARA-R410M-02B with native serial UART

[Alexander Dahl]

Hei hei,

on my desk I have a Microchip SAMA5D27-SOM1-EK1 development board with two mikroBUS expansion ports, in one of them a mikroe LTE IoT Click board is plugged which has a u-blox SARA-R4 series LTE-M modem module on it.

The hardware is okay, I could connect the modem board to a PC with its USB port and ModemManager could use it (somewhat, still waiting for a LTE-M enabled SIM card).

However on the target platform, the modem module is not connected trough USB but trough an ordinary serial UART. And: that serial port is not provided by a serial-usb-adapter, but is a native embedded serial uart, the platform driver is 'atmel_usart_serial'.

I can open a terminal (picocom) and send AT commands manually over that port (/dev/ttyAT2). That works.

Now, I started ModemManager (1.16.2) like this (after disabling and stopping it in systemd):

  MM_FILTER_RULE_TTY_PLATFORM_DRIVER=0 MM_FILTER_RULE_NET=0 ModemManager --debug

The log seems to indicate the port itself is recognized and not filtered, at least that's how I read it:

ModemManager[190]: <debug> [1612137732.891979] [ttyAT2] port contents loaded:
ModemManager[190]: <debug> [1612137732.896216] [ttyAT2]   bus: platform
ModemManager[190]: <debug> [1612137732.900846] [ttyAT2]   device: /sys/devices/platform/ahb/ahb:apb/f8024000.serial/atmel_usart_serial.1.auto
ModemManager[190]: <debug> [1612137732.905038] [ttyAT2]   driver: atmel_usart_serial
ModemManager[190]: <debug> [1612137732.909287] [base-manager] adding port ttyAT2 at sysfs path: /sys/devices/platform/ahb/ahb:apb/f8024000.serial/atmel_usart_serial.1.auto/tty/ttyAT2
ModemManager[190]: <debug> [1612137732.915254] [base-manager] port ttyAT2 is first in device /sys/devices/platform/ahb/ahb:apb/f8024000.serial/atmel_usart_serial.1.auto
ModemManager[190]: <debug> [1612137732.921653] [plugin-manager] task 1: new support task for device: /sys/devices/platform/ahb/ahb:apb/f8024000.serial/atmel_usart_serial.1.auto
ModemManager[190]: <debug> [1612137732.926594] [plugin-manager] task 1: port grabbed: ttyAT2
ModemManager[190]: <debug> [1612137732.931223] [plugin-manager] task 1,ttyAT2: new support task for port
ModemManager[190]: <debug> [1612137732.935370] [plugin-manager] task 1,ttyAT2: deferred until min wait time elapsed

However later on:

ModemManager[190]: <debug> [1612137734.427126] [plugin-manager] task 1: min wait time elapsed
ModemManager[190]: <debug> [1612137734.427829] [filter] (tty/ttyAT2) port filtered: forbidden
ModemManager[190]: <debug> [1612137734.431706] [plugin-manager] task 1: extra probing time elapsed
ModemManager[190]: <debug> [1612137734.432289] [plugin-manager] task 1: no more ports to probe
ModemManager[190]: <debug> [1612137734.432649] [plugin-manager] task 1: all port probings completed, but not reached min probing time yet

And later:

ModemManager[190]: <debug> [1612137735.427471] [plugin-manager] task 1: min probing time elapsed
ModemManager[190]: <debug> [1612137735.428177] [plugin-manager] task 1: no more ports to probe
ModemManager[190]: <debug> [1612137735.428568] [plugin-manager] task 1: finished in '2.506986' seconds
ModemManager[190]: <info>  [1612137735.429067] [base-manager] couldn't check support for device '/sys/devices/platform/ahb/ahb:apb/f8024000.serial/atmel_usart_serial.1.auto': not supported by any plugin

The modem is not listed with `mmcli -L` and is not found `mmcli -S`. I tried to figure out how to tell ModemManager there's an actual modem connected to /dev/ttyAT2 but all I can find in the documentation is about udev rules for usb devices.

What's the missing link here? How can I proceed to get that modem working?

Greets
Alex