Changing the baud rate of additional I2C busses

Question

for some project I need 5 sensors. Their I2C address is fixed and can be changed to another fixed value by soldering. So without an I2C multiplexer (which I don't have), I need at least 3 I2C busses.

Hence, I added two additional I2C busses to my Raspberry Pi 3B by adding the following lines to /boot/config.txt:

# open another i2c bus on GPIO 20 (SDA4) and GPIO 21 (SCL4)
dtoverlay=i2c-gpio,bus=4,i2c_gpio_delay_us=1,i2c_gpio_sda=20,i2c_gpio_scl=21

# open another i2c bus on GPIO 20 (SDA3) and GPIO 21 (SCL3)
dtoverlay=i2c-gpio,bus=3,i2c_gpio_delay_us=1,i2c_gpio_sda=19,i2c_gpio_scl=26

On the breadboard, this works just fine and I receive the data reliably (using an adapted version of these instructions). Now I want to distribute the sensors at quite some distance from the Raspberry Pi using some 50m of data cable. I know that I2C isn't made for this and, in fact, it doesn't work out of the box. Using the built-in I2C bus (bus 1), I immediately get an IOError if I try to read the sensor over the data cable. However, if I alter the baud rate from the standard 100 kHz to 10 kHz by adding

i2c_arm_baudrate=10000

to /boot/config.txt, everything works fine.

My question is: How can I alter the baud rate of the additional I2C busses? Is this possible at all? I tried i2c_gpio_baudrate without success and couldn't find any reference on how to do that.

Thank you very much!

Answer 1

I think I can actually answer the question myself now.

You can change the baud rate of additional I2C busses on the GPIOs by changing the i2c_gpio_delay_us paramter, which defines some time constant in microseconds (check out /boot/overlays/README). The default seems to be 2, which supposedly corresponds to approx. 100 kHz. Changing that to 20 got me a working I2C bus over the 50m of data cable (at approx. 10 kHz I guess).

In short, the lines in /boot/config.txt now read

# open another i2c bus on GPIO 20 (SDA4) and GPIO 21 (SCL4)
dtoverlay=i2c-gpio,bus=4,i2c_gpio_delay_us=20,i2c_gpio_sda=20,i2c_gpio_scl=21

# open another i2c bus on GPIO 20 (SDA3) and GPIO 21 (SCL3)
dtoverlay=i2c-gpio,bus=3,i2c_gpio_delay_us=20,i2c_gpio_sda=19,i2c_gpio_scl=26

and everything works as expected.

Answer 2

If the distance is your concern, why not using a bus buffer dedicated for I2C communication over long distances? Like the P82B96? This can go up to 250m with bus speeds of 120kHz (much more than you apparently need).

The buffer converts the voltage level to a current, sends it to the other end of the cable and then another buffer converts it back from current to voltage.