iPad Camera Connection Kit - gopiballava/iOS-external-interfacing GitHub Wiki

Apple sells the iPad Camera Connection Kit.

It only works on the iPad, not the iPhone. It comes with two adaptors. One accepts an SD card, the other has a USB type A socket. The official purpose of the device is to allow you to download photographs from your digital camera.

While it doesn't appear to be an advertised feature, the CCK allows you to use:

• Keyboards
• Audio input and output devices
• MIDI interfaces

MIDI interfaces in particular are quite interesting because they are full, bidirectional serial interfaces. Using CoreMIDI on the iPad, you can talk to a MIDI device. If you use a Teensy, you can easily communicate with code running on an AVR.