Make I²C initialisation (Wire.begin()) not slow

[matthijskooijman]

I'm running the Wire library on a G030 and noticed that initialization is super slow (Wire.begin() takes 800ms).

Digging into the code, I found i2c_computeTiming() that contains 5 (!) nested forloops and essentially seems to just bruteforce all possible timing combinations or something like that.

Note that I tested on an older version of the Wire library, but looking at the history of that file, I do not think this was fixed in the meanwhile.

Describe the solution you'd like

It seems like it should be possible to be smarter and also a lot faster about this.

[fpistm]

Hi @matthijskooijman
Yes it is slow mainly on small device and it is mentioned in the Wiki:
https://github.com/stm32duino/wiki/wiki/Custom-definitions#i2c-timing
Simply define the I2C timing to avoid call this function as explained in the Wiki.
https://github.com/stm32duino/wiki/wiki/Custom-definitions#example-for-a-stm32f0xx-using-hsi-clock-as-i2c-clock-source-in-varianth