g 4 and 5, not an additional I2C interface), and also added two pins next to RESET, one is IOREF, to allow the expansion board to adapt to the onboard voltage. This pin just tells the expansion board what the current onboard voltage is (for example, UNO is 5V, which can be regarded as a copy of the power supply pin, and does not provide level pull-up), and the other is for the future A placeholder pin to be used.
More stable RESET circuit. The position of the RESET button has also changed, a