I2C: the baudrate seems to be capped.

[nspsck]

So here is the thing, the baudrate seems to be capped at 336000 even ST states that the F446 can go as far as 1MHz. What could be the causes for this issue? I am using the official board configurations.

MicroPython v1.22.0-preview.32.g86c7b957a.dirty on 2023-10-20; NUCLEO-F446RE with STM32F446xx

Type "help()" for more information.

>>> pyb.info()
ID=35001700:03503441:39363120
DEVID=0x0421
REVID=0x1000
S=168000000
H=168000000
P1=42000000
P2=84000000
_etext=806683c
_sidata=8066844
_sdata=20000000
_edata=20000034
_sbss=20000034
_ebss=20000ba8
_sstack=20017ff8
_estack=2001bff8
_ram_start=20000000
_heap_start=20000ba8
_heap_end=20017ff8
_ram_end=2001c000
qstr:
  n_pool=2
  n_qstr=24
  n_str_data_bytes=236
  n_total_bytes=716
GC:
  93120 total
  7968 : 85152
  1=147 2=23 m=40
LFS free: 95744 bytes
>>> i2c = pyb.I2C(1, baudrate=1000000)
>>> print(i2c)
I2C(1, I2C.CONTROLLER, baudrate=336000)
>>> 

[nspsck]

After some digging it came across to me, that the maximum of 400KHz is only achievable when PLCK1 is a multiple of 10MHz. Which means, it's impossible to get, if we want a clock frequency of a multiple of 48MHz for the sake of the USB connection. (I am aware that this is not a very detailed argument and it can well be wrong.)

[jimmo]

I wondered if it might be something like this when I saw your original post, but didn't have a chance to investigate yet, glad you figured it out. FWIW, have you seen ST's interactive clock configuration tool (in STM32CubeMX), it's quite useful for answering these sort of questions.

[nspsck]

Thank you for the information!

Sadly, information such are not easily obtained directly in the CubeMX. The best hint I've got is that the I2C Clock can not go higher 0.4 Mhz. And that was from I2C parameter settings...