Level Shifters are used in multi voltage design in which more than one voltage supply used. Consider In your design two voltage domains are there. One voltage domain V1 has 1.2V power supply another domain V2 has 1V power supply. Signal has to cross from one domain to another domain while in functional mode.
Now assume signal crossing from Low voltage domain V2 to High voltage domain V1, It’s logic is interpreted wrongly at V1. To prevent this level shifters are inserted between the different voltage domains for the signals which cross from low voltage domain to High voltage domain and from High voltage domain to low voltage domains.
The main functionality of the level shifters is to shift the voltage one voltage to another voltage level depending upon the the signal crossing different voltage domains.
Isolation cells are used between the domains. Consider there are two domains are in your design i.e,. D1 and D2. The domain D1 is power shut down mode and other domain D2 is in active mode. Since Domain D1 is power down mode it can propagate invalid logic to domain D2. To prevent this, Isolation cells are inserted between the domains to clamp a known value at its output, While domain D1 is shut down mode. Isolation cells should be placed in always on domain to serve it’s functionality (clamp the known value to the other domain).
Power switches are used in power gating technique. As we already discussed in previous post, Power gating is used to reduce the static (Leakage) power in the design. Power gating is performed by shutting down the power for portion of design. Power switches are used to turn off the portions of design which are inactive at a point of time to reduce leakage power.
Retention Registers are used to store register states before power down mode. These values will be restored when power is up. So retention cells should be always on to serve the purpose. As these are always on, It can consume power even power down mode.
Always On cells:
These cells are special cells which should be always on to their purpose.