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From a software and hardware standpoint, what kind of interface does the keyboard use? I'm guessing it's not just a usb-hid interface, or if it is there's something special for the special keys.

If I were to make my own device like that, say based on an Android sbc, what would I have to do to replicate it?

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Why would there need to be something special for the special keys? All the keyboard does is send an integer keycode when a key is pressed, special or otherwise. It's the software that's responsible for turning the keycode into an event such as "insert x" or "return home". –  Trebor Rude May 2 '13 at 7:54
    
So then it's just USB-HID with a custom keyboard layout? –  mordac May 2 '13 at 18:47
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It appears to be an I2C interface. According to this post on xda-developers asusdec is the name of the keyboard dock device. When I cat /proc/bus/input/devices on my Transformer, this is what it says about asusdec:

I: Bus=0000 Vendor=0000 Product=0000 Version=0000
N: Name="asusdec"
P: Phys=/dev/input/asusdec
S: Sysfs=/devices/platform/tegra-i2c.1/i2c-1/1-0019/input/input15
U: Uniq=
H: Handlers=sysrq event3 
B: PROP=0
B: EV=20003
B: KEY=3fffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff fffffffe
B: LED=2

Also, ps shows a com.asus.keyboard process running as system. That's an Android process. I'm not sure how exactly it's getting the input from the keyboard, though. I assume it has sufficient permissions to open the device for reading and converts the incoming keycodes to Android events.

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