This repo shows how to add an unique identifier (ASCII-string) to the cheap CH340G USB-to-UART modules. It contains informations about the needed hardware-hacking and a hacked driver for Linux.
Indeed, but it has status pins that can be repurposed/abused to bitbang I2C from inside the driver.
Yes, definitively. It is intended as a proof of concept only. You probably should just use USB-to-UART modules that have a "builtin" unique serial number. The Linux-Kernel is a really complex animal and i have basically no idea what i am doing [insert meme here]...
Well, i had some CH340G modules in use and needed a way to distinguish them when plugged into random USB ports. I knew about those status pins because i used them as GPIO already, the Linux driver is open-source (Thanks to all the developpers!) and i had an old 24C16 (I2C-EEPROM) lying around, so i thought "Would it be possible?" and got carried away...
Also while there are other modules like PL2303 or FT232H, they are more expensive and/or the IC have smaller packages that makes it more difficult to add an EEPROM. The CH340G on the other hand comes in a handy SO-package with 1,27mm spacing (opposed to 0,5mm on the PL2303 e.g.).
The modified driver is under the GPLv2 as the original.
The tool for writing/reading the EEPROM using libusb is under AGPLv3+.
The pictures (todo), schematic and text are provided under Creative Commons CC BY-SA 4.0.
As usual the content of this repo is provided WITHOUT ANY WARRANTY! USE AT YOUR OWN RISK! It is really, really experimental stuff. Messing with Kernel-modules involves risks like crashes or worse. If you want to test this code do it inside a VM with USB-passthrough or at least not on a production machine please!
You need an 24Cxx (currently the code is written for 24C16, i don't know if smaller/bigger EEPROM will work out of the box as i am still waiting for my order and didn't look at the datasheets carefully), a 100nF ceramic capacitor for the EEPROM, a pull-up resistor (something like 3k-10k should be fine) and a shottky-diode (like BAT54).
Picture(s) may follow once i received my order.
Really simple, starting at address 0x0000:
- 2 bytes signature to identify the EEPROM: ASCII 'C', 'H'
- identifier as null-padded ASCII-string. Maximum length is adjustable by
#definein the driver, currently set to 8 caracters. - (don't care for the remaining space in EEPROM)
The original driver was borrowed from Kernel 6.1.7. The file is linux-6.1.7/drivers/usb/serial/ch341.c. I have no clue if this code can be compiled for 5.x or an even older Kernel.
You need some dependencies like GCC (obviously), make, the Kernel-headers and maybe other stuff. Look on the internet... Once all this is ready just cd into the directory of ch341_eeprom.c and Makefile (it's a single line but needed!) and run make -C /lib/modules/`uname -r`/build M=$PWD. Compiling takes only a few seconds and will give you a Kernel-module called ch341_eeprom.ko in the same directory.
You will need superuser-priviledges to continue.
Create a file /etc/udev/rules.d/99-serial.rules
ACTION=="add", SUBSYSTEM=="tty", ENV{ID_VENDOR_ID}=="1a86", ENV{ID_MODEL_ID}=="7523", ATTRS{identifier}=="$chosen_identifier", SYMLINK+="$wanted_symlink"
[same thing with different identifier and symlink for next device, as often as you need]
and then tell udev to reload everything: udevadm control --reload. The identifier must be an ASCII-string, currently its length is set to 8 characters maximum.
First you may have to unload the original driver by doing modprobe -r ch341. Then you can simply use insmod ch341_eeprom.ko from inside the correct directory. If you get an error about missing symbols load usbserial first by executing insmod /lib/modules/`uname -r`/kernel/drivers/usb/serial/usbserial.ko. To remove the module do rmmod ch341_eeprom.ko.
Do you really want to use this hack? If so it probably involves blacklisting the original driver and somehow adding the new, hacked driver somewhere... I have to do some research... Stay tuned.
I made a small tool that uses libusb-1.0. It is called ch340g_eeprom and needs to be run as root because of permissions. Tested on Debian 11.
As simple as ./make, assuming you have libusb-1.0-0-dev (name of the package on Debian 11) installed.
This is ch340g_eeprom version 0.1 (c) 2023 by kittennbfive
THIS TOOL IS PROVIDED WITHOUT ANY WARRANTY!
WARNING: Only connect a single CH340G to your PC when using this tool!
usage: ch340g_eeprom $command [$identifier]
$command:
--help: Print this text and exit
--version: Print version and exit
--read: read identifier
--write $identifier: write identifier
Caution: As said, only connect a single CH340G to your PC when using this tool! This is because the tool uses libusb_open_device_with_vid_pid() (because i am lazy...) that will open the first matching device, so if there are multiple CH340G connected you will get unpredictable results.