EV3 I2C

The [AM1808 SoC](EV3 Processor) has 2 hardware Inter-Integrated Circuit (I2C) controllers. Additionally, there are 4 software controllers implemented using [fast interrupts](EV3 FIQ) (FIQs).

Usage

I2C0: [Boot and bluetooth EEPROMs](EV3 EEPROM) @400kHz

I2C0: Not used/connected

[FIQ](EV3 FIQ) I2C: Input ports 1-4 @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](EV3 FIQ) (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.

Sensor Addressing

The I2C bus uses a 7-bit addressing scheme (there is also 10-bit addressing but we are not using it). When sending an address over the bus, the address is shifted to the left 1 bit and the LSB is used to indicate read or write. It is important to know this because LEGO and other 3rd-party sensor manufacturer use the shifted value as the address whereas the linux kernel uses the unshifted value as the address.

Know Sensors and Addresses

Address		Sensor	Notes
Shifted	Unshifted
0x02	0x01	LEGO NXT Ultrasonic sensor	This address is illegal according to the I2C spec, so we had to patch the kernel driver to make it work.
0x14	0x0A	Mindsensors Light Sensor Array	The address can be changed programatically