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diff --git a/README.md b/README.md new file mode 100644 index 0000000..e0347d3 --- /dev/null +++ b/README.md @@ -0,0 +1,211 @@ +# LTE-M Dongle + +I am increasingly using my home automation system, and I wanted a backup +connection in case my primary Internet connection fails. Since I plan to place +it on a small UPS, this will also address power failure scenarios. The obvious +option would be to use a 3G/4G/5G radio modem, but it seems like overkill for, +hopefully rare, SSH access to a remote computer. In addition, such modems often +have a weak internal antenna for indoor reception, and lack an easy way to add +an external antenna. + +After exploring various alternatives, I identified an LTE-M modem as a suitable +solution. It offers better sensitivity than a standard LTE modem, operates on +low power (good for UPS usage), and while it doesn't offer high throughput, it +is more than sufficient for my intended use. On the downside, many LTE-M modems +only provide a serial port for sending vendor-specific AT commands to open UDP +or TCP connections, and I wanted to avoid developing dedicated software to +interface with my home automation system. Fortunately, looking at the various +available LTE-M modules, I have discoverd the SIMCOM SIM7080G module, which is +based on a Qualcomm modem, and feature a high-speed USB interface that can +appear as a network interface on my Linux system. That led me to design my own +LTE-M dongle with an SMA connector to accommodate an external antenna. + + +## Photos + +[](images/lte-m-dongle-top.jpeg) +[](images/lte-m-dongle-bottom.jpeg) + +[](images/lte-m-dongle-enclosure.jpeg) + + +## Hardware + +The board was designed using [KiCad](https://www.kicad.org/) 6.0. The +[schematic](images/schematic.png) is relatively simple, and centers around the +[Simcom SIM7080G](https://www.simcom.com/product/SIM7080G.html) LTE-M and +NB-IoT modem, which is small enough to fit in a [Hammond +1551USB2](https://www.hammfg.com/electronics/small-case/plastic/1551usb) +enclosure, my standard choice for dongles related to home automation, while +still hand solderable. The high-speed USB interface is connected to the USB +connector through the corresponding ESD protection and power filtering, while +the UART, I²C, SPI and PCM interfaces are left unconnected. The GNSS feature +can not be used simultaneously with the LTE-M connection, so it is unused in +this dongle, and the corresponding antenna is left unconnected. The module +requires a 3.8 V power supply. It is generated from the 5 V USB through a +[TPS62A01](https://www.ti.com/product/TPS62A01) buck converter for higher +efficiency. A nano-SIM card connector is also connected to the module, along +with two LEDs providing status feedback: LD1 for the network status and LD2 for +the module status. + +Two solder jumpers are also present, one to switch the module to firmare update +mode, and the other one for the power-up configuration. The latter jumper +deserves some additional explanations. The module is off by default, and can be +powered on by briefly pulling the PWRKEY pin to ground. However this pin can +not be shorted all the time to GND as the module switches off when this pin is +pulled down for approximately 12 seconds. To ensure the module remains +permanently on, a trick is used: connecting that pin through a resistor to the +1.8 V power supply generated internally by the module. When the module is off, +this voltage is close to 0 V, powering on the module. Once the module has +started, the 1.8 V power is available, and thus the pin is no longer pulled +low. This also means the module will automatically restart after being powered +off. This feature is selectable by a jumper: either short pin 1 and 2 together +to keep the module running permanently or connect an external system shorting +the pins 2 and 3 to control the power of the module. + +The PCB is a four-layer design, required to provide 50-ohm impedance for the RF +trace and the 90-ohm differential impedance for the high-speed USB traces. The +bottom side accomodates the nano-SIM connector and the jumpers, making them +accessible by removing two screws from the enclosure. The top side houses the +SIM7080G module and everything else, including the two LEDs, visible through +the transparent enclosure. + +All the components from the [bill of material](BOM.txt) should be easily +available from many distributors. Soldering of the SIM7080G module requires a +hot air gun, but the LGA style pads are relatively large, so it's relatively +easy to do. I personally didn't use a stencil. + + +## Software + +### Useful AT commands + +When connected on an USB port, the modem provides four CDC ACM serial +interfaces, appearing as `/dev/ttyUSBx` on Linux. The AT commands can be sent +on the third and four interfacs. Here are a few useful AT commands to check +that the LTE-M modem work correctly: + +- Display the manufacturer of the module +``` +AT+CGMI +SIMCOM_Ltd + +OK +``` + +- Display the model of hte module +``` +AT+CGMM +SIMCOM_SIM7080 + +OK +``` + +- Display the firmware revision of the module +``` +AT+GMR +Revision:1951B07SIM7080 + +OK +``` + +- Display the product identification information of the module + +``` +ATI +R1951.04 + +OK +``` + +- Display the International Mobile Equipment Identity (IMEI) of the module + +``` +AT+GSN +8600xxxxxxxxxxx + +OK +``` + +- Check if PIN code is required +``` +AT+CPIN? ++CPIN: SIM PIN + +OK +``` + +- Entering PIN code if required +``` +AT+CPIN=1234 +OK +``` + +- List available operators +``` +AT+COPS=? ++COPS: (1,"F-Bouygues Telecom","BYTEL","20820",9),(1,"F-Bouygues Telecom","BYTEL","20820",7),(1,"F SFR","SFR","20810",9),,(0,1,2,3,4),(0,1,2) + +OK + +``` + +### Internet connection + +The modem should be recognized directly by the Linux system, it provides a CDC +Ethernet interface. Configuring it with DHCP already provides an Internet +access. However I prefered to use a PPP connection over the CDC ACM interface +as it allows for monitoring the availability of the Internet connection. + +To do this on a Debian Bookworm installation, I installed the +[`ppp`](https://packages.debian.org/bookworm/ppp) package. The default `gprs` +script works fine after replacing `/dev/modem` by the serial device +corresponding to the USB modem. The modem provides multiple CDC ACM interface, +in my case I used the fourth interface through the +`/dev/serial/by-id/usb-SimTech__Incorporated_SimTech_SIM7080_1234567890ABCDEF-if03-port0` +device. Addressing the device by ID is also useful, as I have other USB devices +with serial interface on the host, and their order is not stable. However, it +seems that the serial number is not properly filled by the manufacturer, so +it's probably not possible to use the two of these modems of the same host. + +To start the start the Internet connection automatically when the modem is +plugged into the USB port, and to restart it automatically in case of +connection failure, I used the following systemd unit: +``` +[Unit] +Description=PPP Daemon +ConditionPathExists=/dev/serial/by-id/usb-SimTech__Incorporated_SimTech_SIM7080_1234567890ABCDEF-if03-port0 +After=network.target + +[Service] +ExecStartPre=/bin/sleep 1 +ExecStart=/usr/sbin/pppd nodetach linkname ppp0 call provider +KillMode=mixed +Restart=on-failure +RestartSec=30 +RestartPreventExitStatus=SIGKILL + +[Install] +WantedBy=multi-user.target +``` + +Once the connection is started, LD1 should blink with a 300 ms period. + +### Backup connection strategy + +My LTE Internet provider only provides a private NATed IPv4, so I use a +[Wireguard](https://www.wireguard.com/)-based VPN to be able to access the my +home automation system remotely. Since I do not want the LTE connection to be +the default one, I added a `nodefaultroute` option to +`/etc/ppp/peers/provider`, and I instead added a manual route to my Wireguard +server. Therefore as long as at least one of the two Internet connections +(fiber and LTE) is available, my home automation system is accessible remotely. +For the outgoing traffic, I also added a manual route to my mail and SMS +servers to ensure they are routed through the Wireguard VPN, so alert messages +can always be sent. + + +## License + +The contents of this repository is released under the [Creative Commons +Attribution-ShareAlike 4.0 International License (CC BY-SA 4.0)](LICENSE). |