Today, there are many 3-V logic families available. With the introduction of even lower voltage standards, such as 2.5 V and 1.8 V, it can be expected that mixed voltage interfacing issues will be around for many years. There are pitfalls to watch out for when interfacing mixed-voltage systems. These pitfalls can be avoided by paying close attention to a few key issues. These include the maximum voltages applied to the input and output pins, the current flowing between the power supplies, and the input-switching threshold levels that must be met.
Some circuits have limitations on the voltage that can be applied to an input or output pin.
These circuits can have current paths to through diodes or parasitic elements. If the voltage is high enough, current will flow into the device to the 3-V supply. Excessive current can also damage the diodes and circuitry. Either of these situations will cause data disruption and may damage components. A mixture of TTL and CMOS switching levels can exist too.