Host adapter

Andrea Barisani edited this page Jul 12, 2016 · 8 revisions

Enabling host mode

The USB armory is primarily meant to be attached to a USB host, such as a laptop or desktop computer. However, it can be also used as a standalone computer by using a custom host adapter.

The host adapter leverages the fact that it is possible, by changing the Linux kernel device tree configuration, to invert the role of the USB On-The-Go port currently used as main plug for the board. This allows the USB armory to be used independently as a host.

The role change can be enabled by using a device tree source file (dts) with the following configuration:

&usbotg {
        dr_mode = "host";
        status = "okay";

The official USB armory repository provides an example here.

The device tree binary file can then be compiled as shown in the microSD image preparation instructions, taking in account the different dts file.

The resulting dtb can be copied to /boot/imx53-usbarmory.dtb on the USB armory when host mode functionality is desired (it is recommended to keep the standard device dtb file around to switch it over when desired).

Hardware adapter

In order to use host mode an adapter is needed to perform the following functions:

  • Bridging the USB armory male plug to a USB Type A receptacle (gender changer).
  • Accepting power from a micro-USB input.

This simple conversion enables connection between the USB armory, power supply, and a USB device or hub. Therefore, it is only a matter of compiling the right Linux kernel modules and the USB armory can independently use a keyboard, USB display, USB mass storage devices, USB WiFi dongle and more.

Connecting a powered USB hub to the adapter ensures that all the connected USB devices have enough power to perform their tasks. Additionally, a micro-USB cable we can power the USB armory itself. Alternatively, a passive USB hub can be used and a micro-USB charger (such as ones used for most mobile phones) can provide power.

The host adapter can be easily built using a breadboard as follows (only recommended for people with experience in DIY hardware and electronics):

  • D+, D- bridged from a Type A receptacle to the other
  • VBUS, GND shorted between the two Type A receptacles and a micro-USB one

The adapter is available for purchase along with the USB armory board, please see information at the USB armory project page.

Fun fact: the host adapter can also be used to prevent accidental data exchange enabling secure charging.

USB armory host adapter