stm32f103xx: I2C has no internal pullups and af config

[MrKevinWeiss]

Description

When using the nucleo-f103rb the standard configuration is i2c0 on PB8 and PB9 the AFIO periph clock must be enabled
RCC->APB2ENR |= 0x00000001;
and the pin must be remapped to those pins
AFIO->MAPR |= AFIO_MAPR_I2C1_REMAP;
Also due to the hardware constraints it seems like the internal pullup cannot be enabled since the pins must be set to GPIO_AF_OUT_OD.

In the reference manual it seems like it is impossible to do so (in section 9.1.9 Alternate function configuration it states that 'The weak pull-up and pull-down resistors are disabled.')

I am not sure if anything can be done about requiring external pullups.

Steps to reproduce the issue

Try to use the i2c0 on any nucleo103xx on PB8 and PB9.

Expected results

It works.

Actual results

It doesn't work and will hang

[MrKevinWeiss]

@aabadie, @haukepetersen, or @dylad, I would like your input on how to cleanly add an remap config and if you know any way to try to activate the pullup.

[maribu]

The same issue affects the blue pill (and clones). Using PB6 and PB7 instead is my proposed solution in that case, because than no remapping of the first I2C interface is required. Would that be an option for the Nucleo as well?

[MrKevinWeiss]

Ya but then you would have to use some morpho pins (I think) and not the nice labeled/standard sda and scl. I think it should eventually be fixed in the CPU as those pins should be usable.

When I find I can try to figure something out, I just didn't want it forgotten.