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Tool to control an usb-sd-mux from the command line

Project description



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.


Clone the git repository:

$ git clone

Create and activate a virtualenv for usbsdmux:

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

Install usbsdmux into the virtualenv:

$ python install

Now you can run usbsdmux command by giving the appropriate /dev/sg* device, e.g.:

$ usbsdmux /dev/sg1 dut
$ usbsdmux /dev/sg1 host

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 with something like sudo /path/to/virtualenv/bin/usbsdmux /dev/sg1 DUT.

Using as non-root user

Access to /dev/sg* needs the CAP_SYS_RAWIO. By default all processes created by root gain this capability.

Since you do not want to give this capability to the Python interpreter you

  • either need to call the scripts as root
  • or use the systemd-service.

The systemd-service is intended to be used with socket-activation. The service is present inside usbsdmux-service.

The systemd-units provided in contrib/systemd/ show an example of how to set up the service with systemd and socket-activation. You may adapt and copy them into your machine’s local systemd service folder /etc/systemd/system/

To start the socket unit and let it create the required socket path (requires permissions), run:

systemctl start usbsdmux.socket

Now you can use the usbsdmux tool from a non-root user by calling it with the client -c argument, e.g.:

usbsdmux -c /dev/sg1 DUT

If you use a non-standard socket path (i.e. not /tmp/sdmux.sock) you also need to explicitly set the socket path:

usbsdmux -c -s /path/to/sock.file /dev/sg1 DUT

Reliable names for the USB-SD-Mux

A USB-SD-Mux comes with a pre-programmed serial that is also printed on the device itself. With the udev-rule in contib/udev/99-usbsdmux.rules the sg-device for every USB-SD-Mux is linked to a device in /dev/usb-sd-mux/id-*.

This makes sure you can access a USB-SD-Mux with the same name - independent of the order they are connected or the USB or the USB-topology.


  • Access to /dev/sg* needs the CAP_SYS_RAWIO. The service should drop all not needed capabilities after it is started.

Project details

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