The different potential interface of the RL78 Family enables signals to be exchanged between the RL78 and a device operating at a lower power supply voltage.
There are two methods for using it.
One method is to make the high-level threshold of the input pin lower than the normal CMOS input, and lower to the same level as TTL or below. This allows the low high-level output voltage of the connected device to be supported (see below figure) while at the same time making the output pin N-ch open drain, preventing voltage greater than the power supply voltage from being applied to the connected device. Because the I/O buffers differ for this method, it is mainly used for serial interface signals. The other method is supported by the RL78 products that have many pins. The power supply (EVDD) of the pins is independent from the CPU power supply (VDD). In this case, signals can be exchanged between devices operating on different voltages by making the EVDD voltage the same as the the power supply voltage of the connected device.