The AM1808 SoC has 2 hardware Inter-Integrated Circuit
(I2C) controllers. Additionally, there are 4 software controllers implemented using
fast interrupts (FIQs).
Usage
I2C0: Boot EEPROM at 400kHz
I2C1: Not used/connected
FIQ I2C: Input ports 1-4 at 10kHz
Device Drivers
The Linux kernel has a well established framework for I2C drivers.
The SoC I2C uses the existing driver at drivers/i2c/busses/i2c-davinci.c as a backend.
Since there are not enough I2C controllers on the SoC, LEGO chose to implement
the I2C buses for the four input ports using GPIOs. A secondary reason for this
is that the NXT Ultrasonic sensors does not work well with standard I2C masters.
It requires an extra clock pulse between the stop and the next start. (TODO: add
references here.)
This Linux kernel has an existing GPIO i2c driver, but in order to get the
performance needed, we have to use fast interrupts (FIQs). (Be sure
to follow the link for some interesting reading.) So, the really low level stuff
is done in arch/arm/mach-davinci/legoev3-fiq.c and the rest is in
drivers/i2c/busses/i2c-legoev3.c.
I2C Addressing
See I2C Sensor Addressing.
.