Just in time for 2013, I hooked up the smart meter which was installed a month ago:
This connects DIO4 to the output of the P1 port, which has the following pinout:
The circuit is as follows:
The request pin has to be pulled to 5V (it very likely just powers the isolated side of the built-in optocoupler). The 10 kΩ signal pull-up is needed to improve the rising flank of the signal, and the 10 kΩ resistor to the I/O pin prevents problems when the input signal rises above the 3.3V powering the ATmega.
Tomorrow, I’ll describe the software that goes along with this.