FAQ: What is the default I2C bus clock speed? Can I change it to 400kHz?

[Lazar Demin]

Default Clock Speed: 100kHz

The hardware I2C bus on the Omega2 family has a default clock speed of 100kHz, the standard clock speed for I2C:

How is the I2C Clock Set?

The clock speed is governed by the clock divider in the I2C SM0CTL0 register.

The register's default value is 0x8190 800F, with 0x190 set for the clock divider.

• 0x190 = 400
• The master clock is 40 MHz
• 40 MHz / 400 = 100 kHz

Changing to Fast Mode: 400kHz

It is possible to change the I2C bus clock speed to 400kHz (fast mode) by writing to the SM0CTL0 register using devmem to change the clock divider:

devmem 0x10000940 32 0x8064800F

This changes the clock divider to 0x64 (100), 40MHz / 100 = 400 kHz

To optionally confirm, read the register:

devmem 0x10000940
0x8064800F

Tested on firmware v0.3.2

Credit goes to @György-Farkas from this post from 2018.

More Info

Check out the MT7688 SoC datasheet to see the full register definition.

[Nelson62]

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