Finally I had the time to finish out the developer board that I’m using to program the ESP-01 chips as I described on this link: ESP8266 Developer board.
One of the remaining doubts that I had was if the I2C bus would work at all on the developer board. The issue is that while GPIO2 pin is not connected to anything, it just has a week pull up resistor, the GPIO0 pin is also used to control the firmware flashing through the CP2102 USB-Serial DTR line as we can see on the below schema:
So the GPIO0 pin has two functions, as the SCL (clock line) and as the flashing enable line.
For the Sming framework, if not specified, the GPIO0 is the SCL line, and GPIO2 is the SDA line.
Well with or without the USB cable from the CP2102 connected to the computer, the I2C bus works fine as expected 🙂 and I can detect and control an I2C connected HD44780 lcd display, that worked right out of the box.
The HD44780 works at 5V and so its connected to the ESP8266 I2C bus through a voltage level shifter.