I2C frequencies below 100kbps

[nategreco]

Due to board design flaws that led to insufficient rise time for 100kbps communication, I would like to configure my i2c node in the device tree with a baud rate of 10 kbps.  At build time I get the following build failure:

C:\ncs\v2.1.0\zephyr\include\zephyr\toolchain\gcc.h:77:36: error: static assertion failed: "Wrong I2C 1 frequency setting in dts"
   77 | #define BUILD_ASSERT(EXPR, MSG...) _Static_assert(EXPR, "" MSG)
      |                                    ^~~~~~~~~~~~~~

This is checked in `i2c_nrfx_twim.c`:

#define I2C_NRFX_TWIM_FREQUENCY(bitrate)				       \
	(bitrate == I2C_BITRATE_STANDARD  ? NRF_TWIM_FREQ_100K :	       \
	 bitrate == 250000                ? NRF_TWIM_FREQ_250K :	       \
	 bitrate == I2C_BITRATE_FAST      ? NRF_TWIM_FREQ_400K :	       \
	IF_ENABLED(NRF_TWIM_HAS_1000_KHZ_FREQ,				       \
	(bitrate == I2C_BITRATE_FAST_PLUS ? NRF_TWIM_FREQ_1000K :))	       \
					    I2C_NRFX_TWIM_INVALID_FREQUENCY)

I suspect the answer is no but just want to double check before I wait on building another board - Could the driver support a lower baud rate if I modified this section of code?

Details:
nRF Connect SDK - v2.1.0

nRF52840

[AlanIoT]

I had read multiple forum posts looking for a solution to the same problem, before giving up. Later I stumbled onto this post, which explains a very easy hack to set the bitrate lower than 100kbps (Thank you Burt !)

Under `toolchainVersion > modules > hal > nordic > nrfx > mdk > nrf9160_bitfields.h` you can find the line

#define TWIM_FREQUENCY_FREQUENCY_K100 (0x01980000UL) /*!< 100 kbps */

Simply changing this value according this instruction was the solution to our problem. So I set the value to 10 kbps

#define TWIM_FREQUENCY_FREQUENCY_K100 (0x0028F000UL) /*!< 100 kbps */

and here you can see that the SCL ticks are 100 µs long and the sensor responds now.

We are reading a sensor hanging on an 8 meter long cable, which is impossible without lowering the bitrate to a much lower value.