Second I2C bus on Raspberry Pi running FreeBSD

The Raspberry Pi Rev 2.0 has I2C1 bus on the expansion header, marked as P1.
SCL1 [BCM2835/GPIO3] routed to P1 pin 5
SDA1 [BCM2835/GPIO2] routed to P1 pin 3
But I2C0 bus is still available.

Here is a description how to use second (or first) I2C bus on the Ruspberry Pi running FreeBSD.

SCL0 [BCM2835/GPIO1] routed to S5 pin 13
SDA0 [BCM2835/GPIO0] routed to S5 pin 14
S5 is the CSI camera interface.

But Rev 2.0 has a new connector P5.
This are 8 holes near the expansion header P1.


This carries the four GPIO signals [BCM2835/GPIO28 – BCM2835/GPIO31], along with +5V0, +3V3 and two 0V.

This GPIO allocation provides access to one of:

  • SDA0, SCL0 (Operating independently of P1 SDA1, SCL1); or
  • Four GPIO signals.

So, we can use I2C0 on P5.

But first we need to solder any header for this holes.
P5 is right up against the main GPIO header (P1).
This is a problem, because cable on P1 will compete with the header on P5.
I cut off the tab on the cable connector.

After that we can connect any device to the new header on P5.



If we try to use iic0 device now we will fail.

If we look at source code of the BCM2835 BSC we will see pins 0, 1 for the I2C0 (BSC0).
But we don’t need to change this and recompile the kernel.
BCM2835 BSC driver just sets this pins to mode alt0 (see lines 278-279)

In the Device tree source we can see bsc0_a on pins 0, 1 and bsc0_b on pins 28, 29.

It’s time now to configure FreeBSD to use I2C0 on P5.

Let’s check the mode of pins 0, 1, 28 and 29:


As we see pins 0, 1 are in alt0 mode and pins 28, 29 are in input mode.
To change mode we can use the sysctl(8) utility:


But this is not all.
We need to use pull-up resistors for I2C pins.
They can be external or can be configured internal by the gpioctl(1) utility:
gpioctl -f /dev/gpioc0 -c 28 PU
gpioctl -f /dev/gpioc0 -c 29 PU


Now we can try to use I2C0. I connected the DS1307 RTC device:


It’s works!

To make this settings permament we need to update /etc/sysctl.conf:


And to create a startup script to configure internal pull-up resistors:


And now an example of two I2C devices connected to both I2C buses.


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