What is an enable level shifter (ELS)?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
An enable level shifter is a single cell that combines the functions of isolation and level shifting, so one cell both clamps the output to a safe value when the source domain is off and translates the voltage level when the source domain is on.
Mentor Explanation
An enable level shifter combines isolation and level shifting in one cell, and is placed in the 'on' domain. A typical library excerpt for such a cell (A2LVLD_X1) carries 'is_level_shifter:true', 'level_shifter_type:HL', an enable pin marked 'level_shifter_enable_pin:true', and an output function '(A&EN)' — meaning the output is simply forced low whenever the enable is deasserted, exactly like an AND-type isolation cell, while also performing the voltage translation. Library-requirement rules confirm this dual role: enable-type level shifter cells need both 'is_level_shifter' and 'level_shifter_enable_pin' attributes, and an enable level shifter can substitute for a dedicated isolation cell if that's what the mapped library provides.
Example
At a boundary where a block both changes voltage and needs isolation on shutdown, using one ELS cell instead of a separate level shifter plus isolation cell saves area and simplifies the boundary.
Why It Matters
This question tests whether a candidate understands that these two power-management functions aren't always separate physical cells — libraries often merge them for efficiency.
Common Beginner Mistake
Assuming every voltage-crossing-plus-shutdown boundary needs two discrete cells; many real libraries provide a single combined ELS cell instead.
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