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Welcome to usbsdmux



This software is used to control a special piece of hardware called usb-sd-mux from the command line or python.

The usb-sd-mux is build around a Microchip USB2642 card reader. Thus most of this software deals with interfacing this device using Linux ioctls().

This software is aimed to be used with Labgrid. But it can also be used stand-alone or in your own applications.

High-Level Functions

usbsdmux provides the following functions:

  • Multiplexing the SD-Card to either DUT, Host or disconnect with usbsdmux
  • Writing the Configuration-EEPROM of the USB2642 from the command line to customize the representation of the USB device: usbsdmux-configure

Low-Level Functions

Under the hood this tool provides interfaces to access the following features of the Microchip USB2642:

  • Accessing the auxiliary I2C bus with write and write-read transactions with up to 512 bytes of payload using a simple python interface.
  • Writing an I2C Configuration-EEPROM on the configuration I2C. This is done using an undocumented command that was reverse-engineered from Microchip's freely available EOL-Tools.


Create and activate a virtualenv for usbsdmux:

$ virtualenv -p python3 venv
$ source venv/bin/activate

Install usbsdmux into the virtualenv:

$ pip install usbsdmux

Now you can run usbsdmux -h to get a list of possible command invocations:

$ usbsdmux -h
usage: usbsdmux [-h] SG {get,dut,client,host,off}

positional arguments:
  SG                    /dev/sg* to use
                        get - return selected mode
                        dut - set to dut mode
                        client - set to dut mode (alias for dut)
                        host - set to host mode
                        off - set to off mode

optional arguments:
  -h, --help            show this help message and exit

Using as root

If you just want to try the USB-SD-Mux (or maybe if it is just ok for you) you can just use usbsdmux as root.

If you have installed this tool inside a virtualenv you can just call the shell-wrapper along with the appropriate /dev/sg* device path:

sudo /path/to/virtualenv/bin/usbsdmux /dev/sg0 dut
sudo /path/to/virtualenv/bin/usbsdmux /dev/sg0 host

Using as normal user / Reliable names

The example udev-rule in contib/udev/99-usbsdmux.rules serves two purposes:

  • Allow users currently logged into the system and users in the plugdev group [1] to access connected USB-SD-Muxes.
  • Create a reliable path in the filesystem to access specific USB-SD-Muxes based on their pre-programmed unique serial number. This is useful when multiple USB-SD-Muxes are connect to a system, as the enumeration-order, and thus the /dev/sg* numbering, may differ between reboots. The serial number is printed on a label attached to the device.

Users of a Debian based distribution [1] can install the udev rule by cloning this repository and copying it to the appropriate location and reloading the active udev rules:

$ git clone ""
$ sudo cp usbsdmux/contrib/udev/99-usbsdmux.rules /etc/udev/rules.d/
$ sudo udevadm control --reload-rules

After reattaching the USB-SD-Mux you should get a list of connected USB-SD-Muxes, based on their unique serial numbers, by listing the contents of the /dev/usb-sd-mux/ directory:

$ ls -l /dev/usb-sd-mux/
total 0
lrwxrwxrwx 1 root plugdev 6 Mar 31 11:21 id-000000000042 -> ../sg3
lrwxrwxrwx 1 root plugdev 6 Mar 27 00:33 id-000000000078 -> ../sg2
lrwxrwxrwx 1 root plugdev 6 Mar 24 09:51 id-000000000378 -> ../sg1
[1](1, 2) The plugdev group is used in Debian and Debian based distributions (like Ubuntu and Linux Mint) to grant access to pluggable gadgets. Depending on your Linux distribution you may want to create/use another group for this purpose and adapt the udev rule accordingly.


  • Some single board computers, especially Raspberry Pi model 4s, do not work with new/fast micro SD cards, due to drive strength issues at high frequencies. Use old and slow micro SD cards with these devices. Another workaround is the replacement of resistors R101 and R102 with 0Ω parts. This modifications does however void the EMC compliance statement provided by the Linux Automation GmbH.
  • Some usecases, like hard to reach connectors or full-size SD cards, necessitate the use of adapters or extension cables, leading to the same drive strength issues and require the same workarounds as documented above.
  • In order for the /dev/sg* device to appear the sg kernel module needs to be loaded into the kernel. This is usually done automatically by udev once the USB-SD-Mux is connected. To manually load the kernel module run sudo modprobe sg.


Thank you for thinking about contributing to this project! Changes should be submitted via a Github pull request.

This project uses the Developer's Certificate of Origin 1.1 with the same process as used for the Linux kernel:

Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

  1. The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or
  2. The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or
  3. The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.
  4. I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it, including my sign-off) is maintained indefinitely and may be redistributed consistent with this project or the open source license(s) involved.

Then you just add a line (using git commit -s) saying:

Signed-off-by: Random J Developer <>

using your real name (sorry, no pseudonyms or anonymous contributions).