The new Nokia N810 internet tablet supports a feature called USB-On-The-Go, or shorter USB-OTG. The OTG means that N810 can act in both USB roles, slave and master. You can find more about USB-OTG from http://www.usb.org/developers/onthego/ .

In Slave role N810 acts as a FAT-format memory card or with g_ether driver as a USB networking card. In Slave mode you can connect your tablet to your PC and it shows up by default as external memory card.

Using USB-OTG host mode you can connect your USB memory stick to your tablet, you can also connect tablet to various other memory devices like digital cameras, memory card readers or even to a external hard disk. The N810 can supply limited amount of power to external USB device and no extra powered hub is needed. In practise, hub does not work with the current software version.

To activate USB-OTG host mode there is a special ID pin in micro- or mini USB connector. If this pin is left open, device acts as usb slave(device) mode, if grounded, it acts as master(host) mode. Only problem is that I have not found anywhere a such cable, at least they are not very common in computer shops. If you have a such cable, you can just plug in a USB memory stick and everything works. If you don't have, you have two choices. You can make cable yourself by practicing little bit micro surgery and connect the ID pin to ground or then activate host mode x-terminal shell command prompt.  It is also possible to make small control panel applet to activate the host mode.

Activating host mode from command line

Start "X terminal" application from utilities menu, issue the "sudo gainroot" command in terminal. You should have r&d mode enabled with the linux-flasher to be able use gainroot (sudo ./flasher --enable-rd-mode).

sudo gainroot
echo host> /sys/devices/platform/musb_hdrc/mode

There are also drivers for few other USB devices in the kernel, for example realtek RTL8150 based ethernet adapter.  How to use Realtek ethernet adapter, I will have more info in my next blog entry about USB networking.

If you have come up with some other device that does not yet have kernel support, you may need a separate driver installed for it. As example pl2303 based USB serial converters require an additional driver to be installed. Note also that all devices will not work with N810 even with the driver. N810 can't supply as much power as a PC laptop and USB device power consumption can easilly drain battery empty quickly. All USB devices tells in their device descriptor, how much power they need. If the value is too much, the N810 just refuses to use it or N810 even can't raise power up enough to be able to read this decsriptor. For example there has been used Dlink DUB-E100 with Nokia 770 with external powered USB hub. They are so power hungry that N810 can't use them. Realtek tells that it needs 120mA, D-link 250mA.

Patching normal USB-cable to USB-OTG cable

If you can't find OTG cable anywhere, you can buy normal micro-USB cable and patch it. Don't patch your Nokia cable because OTG-cable can't be used any more to connect your tablet to PC. To patch the cable, you need a small sharp "surgical" knife and soldering iron, and a shrinking tube.

First step is peel soft rubber covering connector body with surgical knife, then lift the top side of metal shield up with knife. You may need to cut the shield soldering that connects the cable to the protective ground. Be careful in lifting the cover because you need to bend it back when the modification is ready. Then solder the two pins together as shown in the picture.