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With the new device tree on my Raspberry Pi 3B+, the i2c baudrate gets set in the /boot/config.txt. Is it possible to set this on the fly instead? I have a device (DAC8574) that needs some initial setup communication at 400 K baudrate first before it can switch to the 3.4 M baudrate.

This is what I have in /boot/config.txt

dtparam=i2c_arm=on,i2c_arm_baudrate=400000

As a test, I tried to rmmod the i2c_bcm2835 and then reload with the following.

sudo rmmod i2c_bcm2835
sudo modprobe i2c_bcm2835 i2c_arm_baudrate=100000

But no luck, the speed remains at 400 K. I'm using the i2c-dev (SMBUS) tools with a C executable. Ideally, I'd like to be able to change the speed within the C program/executable. I could not find anything in the smbus docs about changing speeds. But maybe there is another way.

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    I believe you are more likely to get an accurate answer if you ask on raspberrypi.org/forums. I think there is a device tree section (or used to be). The device tree experts are more likely to see the question there. – joan Jun 15 '18 at 8:50
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Yes. I use RPIO, which has an API to set the I2C baud rate. I have long sensor wires for gardening, and lower baudrates allow me to run longer cables. I set my I2C to 10000 (10kHz for 10meter cables).

RPIO is a Javascript GPIO package. It should be possible to do the same in other languages--consult the documentation of the chosen library for your language.

Caveat: I have not taken my oscilloscope and actually verified that the baud rate changes when so configured. I am simply trusting the provided library API. I would defer to an answer where such research has been conducted. As Joan points out, such information may be available elsewhere.

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