This howto is designed to help using the Arduino code
To download it:
git clone --recursive https://github.com/TakkTile/takktile_arduino.git
Fast I2C Communication
(from here: http://arduino.cc/forum/index.php/topic,16793.0.html)
the atmega-hardware can do 400 KHz, but you have to tweak the Wire-library in file hardware/libraries/Wire/utility/twi.h.
Near the top of the file you see :
If you change that to:
#define TWI_FREQ 100000L
The I²C bus should run at 400kHz
#define TWI_FREQ 400000L
But you also have to the delete the files
This will force the compiler to recompile these files.