Hi WiFive, read the article, seems very interesting. Allocating an external interrupt will always allocate it on the core that does the allocation. https://esp-idf.readthedocs.io/en/latest/api-reference/system/intr_alloc.html Can anyone point me to a example? Just googled for a sort of hands on, bu...

Could you try the latest Arduino ESP build? A rewrite of I2C was merged recently that is significantly better.

It can now be installed via the board manager if you are using Arduino IDE:
https://dl.espressif.com/dl/package_esp ... index.json