May be during ur RTL coding u can insert them as a macro to your design.
we can also put level shifter during floor plan (during specifying power domain) stage if u have failed to put it during RTL coding or synthesis.
whenever a logic is entering from one power domain to another power domain. Lets say u have 4 different power domains 1v 1.3v 1.5v 3v . Whenever any logic traverses from one voltage domain into another, you need level shifter. Many EDA tools are capable of inserting level shifters automatically ( ofcourse u need provide power/voltage domain information in form of constraints or configs etc )
deh_fuhrer said:
In the asic design flow where you'll fit level shifters..
In asic design flow. Inserting levelshifter and isolation cell at synthesis stage is advicable..
That will help you predict all area/timing/power info in earlier stage itself
Modern Implementation tools from Synopsys, Magma and Cadence are all able to insert low power structures such as isolation cells, retention flops and Level shifters are the Place & Route stage.
With the advent of the power format spec language such as CPF or UPF, synthesis tools are able to insert it at the RTL level. My understanding is that Cadence is ahead with the CPF format and many customers have deployed synthesis with CPF so that they can insert Level shifters right at the RTL level. The requirement is that they need a CPF power spec which specifies the power domains of the design.
After the level shifters are added, I would recommend using some sort of formal electrical checking tool such as Conformal - Low Power (CLP) that will verify proper insertion of the level shifters.