Level Shifter Cells

Level Shifters (LS) are special standard cells used in Multi Voltage designs to covert one voltage level to another. 

As Multi Voltage designs have more than one voltage domain, level shifters are used for all the signals crossing from one voltage domain to another voltage domain. 

Like isolation cells, level shifters are inserted by the synthesis tool

Low to High Level Shifters

Low to High LS are used for signals crossing from a lower voltage domain to higher voltage domain.

It basically amplifies the source signal so that it can be interpreted properly in the destination domain. 

Suppose a 0.7V signal is crossing from 0.7V voltage domain to 1.0V voltage domain. Now in the lower voltage domain it corresponds to logic-1 but in higher voltage domain it is neither logic-1 nor logic-0, so the signal becomes unknown (or ‘X’) in the absence of Low to High LS.

High to Low Level Shifters

High to Low LS are used for signals crossing from higher voltage domain to lower voltage domain.
It basically attenuates the source signal. 

Although putting a LS for signals crossing from higher voltage domain to lower voltage domain is optional as signals will be interpreted properly in destination domain, but typically we put a LS to avoid stress on the transistors of lower voltage domain due to high voltage of the source signal.

Bi-directional Level Shifters

When dynamic voltage scaling or dynamic voltage freq scaling is used the voltage relation between the source and destination might change over time of operation. In that case we need level shifter which is capable of shifting both low to high voltage signals and high to low voltage signals.