I2C endianess configuration

[frax84]

Hello,

i'm developing a board that comunicates with two i2c sensors. I developed some i2c routines to use with these sensors. Everything works fine (comunication establish and i can exchange data), but i noticed that the sensors need different endianess in transmission: one needs MSBfirst, the other LSBfirst. In general, i could implement an additional step to swap bytes for one of the two devices, but this would require doubling the functions and it should be considered bad practice if some options is available instead.

I believed that the i2c_dt_spec* spec parameter that i need to pass to:

static inline int i2c_write_read_dt(const struct i2c_dt_spec *spec,
				    const void *write_buf, size_t num_write,
				    void *read_buf, size_t num_read)

could contain a flag (i.e. I2C_BIT_ORDER_MSB)or something to specify the endianess of the comunication but i was not able to find anything in both Nrf Connect SDK and Zephyr documentation.

Is there any info i missed? I'm quite confident that there should be a smart solution for this common problem.

Thank you

Francesco

[JamesDougherty]

I looked over the drivers (linked below) and they have no concept of endianness, they just read and write the data they're provided. This makes sense because the I2C driver is doing it's job, reading and writing the data to and from the I2C device. I presume it's up to the user to determine what byte order the data needs to be in. Zephyr has built in byte ordering functionality that would help with this using the __bswap_* macros (#include <zephyr/sys/byteorder.h> - see https://github.com/zephyrproject-rtos/zephyr/blob/main/include/zephyr/sys/byteorder.h). It would be a nice addition to have them add this flag, like you suggested, so the driver can handle the byte ordering internally. Maybe you should post a feature request on their GitHub.

Base Driver: https://github.com/zephyrproject-rtos/zephyr/blob/main/include/zephyr/drivers/i2c.h

TWI: https://github.com/zephyrproject-rtos/zephyr/blob/main/drivers/i2c/i2c_nrfx_twi.c

TWIM: https://github.com/zephyrproject-rtos/zephyr/blob/main/drivers/i2c/i2c_nrfx_twim.c

[frax84]

Hi James,

thank you for your comment. It seems quite incredible there is no option for such trivial task. I mean, i'm perfectly aware that it's super easy to implement swap byte or using macros like you suggested, but i would have bet that there was an implementation ready for bus protocols. I'll wait for a bit and then post a feature request if nothing comes out.

Thank you,

Frax