Low-Power Tip of the Week: Placement of level shifter and isolation cells

Designers often ask where to place the level shifters and isolation cells.  Should they be in the source or destination domain?  What are the tradeoffs?

The general recommendation is to place these cells in the destination domain.

Isolation cells are typically powered by the destination domain.  If they are placed in the source domain, then the designer must connect them to the destination domain power supply.  This also requires isolation cells to have secondary power pins for the connection to destination domain power supply.

For level shifters, the location generally does not matter because they need to be connected to power supplies of both source and destination domains.  However, some high-to-low level shifters only require the high supply voltage.  In this case, we might want to put them in the source domain.

When an output has a large fanout, then it might make sense to place level shifters and isolation cells in the source domain.  This can significantly reduce the total number of level shifters and isolation cells in the design.

As pointed out in an earlier post, the placement of these cells can be specified in the CPF file.

Luke