tool run CTS. During CTS the tool clones the isolation cells as needed, which handles the clock isolation.

KEY Remove existing CK/RST isolation, convert the last reset buffer to iso-high, and insert iso-low cells on ICG outputs that CTS then clones.

Destination vs Source Isolation Cell

Isolation is needed when a signal crosses from a switchable domain to an always-on domain. The isolation cell can sit in either domain. If it is placed in the switchable domain it is source-isolated; if it is placed in the always-on domain it is destination-isolated.

When the isolation cell is source-isolated (in the switchable domain), it must be supplied with secondary power from the always-on domain so it can keep functioning when its own domain is off.

KEY Source isolation places the cell in the switchable domain (needs AON secondary power); destination isolation places it in the always-on domain.

When Level Shifters Are Needed

Level shifters are needed when there is a significant voltage difference between two domains - specifically when the difference is large enough to exceed the noise margin.

KEY Use level shifters when the voltage difference between two domains exceeds the noise margin.

Body Biasing and Timing

Applying a forward body/substrate bias lowers the threshold voltage Vt. With a lower Vt the device switches faster, so path delays improve and setup closure becomes easier - but the trade-off is higher power consumption.

KEY Forward body bias lowers Vt, speeds the device and eases setup, at the cost of more power.