In order to perform level shift from 1.8V to 2.5V and vice versa.
Level shifts from 2.5V to 1.8V and from 1.8V to 2.5V are shown below:
Level shifts from higher voltage to lower voltage such as from 2.5V to 1.8V can be done by input tolerant function. With input tolerant function, cases such as 3.3V voltage is applied to input when VCC = 0V are allowed and no unwanted current will occur. In addition, the propagation delay time will not decline, so level shift can be done easily.
[Figure: Level shift from 2.5V to 1.8V]
Level shifts from lower voltage to higher voltage such as from 1.8V to 2.5V can be done if output voltage VOH satisfies the input voltage VIH level of the IC in the subsequent stage because VOH≈ power supply voltage VCC when the CMOS input of the subsequent stage is driven by the CMOS output of the IC in the previous stage.
[Figure: Level shift from 1.8V to 2.5V]