What is an enable level shifter (ELS)?
From PDVerse Low-Power Physical Design Mentor Guide, part of the pdVerse Mentor Guide
Short Answer
An enable level shifter is one cell that does both jobs at a boundary: it shifts the voltage while the source is on and clamps the output while the source is off. You use it where a signal leaves a switchable domain and also changes voltage. It saves area and delay compared with a separate isolation cell plus level shifter.
Technical Explanation
- An enable level shifter (ELS) is a level shifter with an enable pin that forces the output to a fixed value.
- The ICC2 MV guide says a cell that does both level shifting and isolation is called an enable level shifter.
- In Liberty it carries
is_level_shifterandlevel_shifter_enable_pinattributes, so the tool knows it can isolate. - ICC2 uses an ELS as the isolation cell when only ELS cells are mapped, or when no plain isolation cell matches.
- The ELS output side runs on the receiving, always-on supply so the clamp survives the shutdown.
- Use one only where the source can shut down; a crossing between two domains that never switch off needs just a shifter.
- Cell names on this page, such as ELS_LH_X1, are illustrative.
Common Mistake
The Trap: Adding a separate isolation cell and a separate level shifter on every switchable, cross-voltage signal.
- You pay two cell delays and extra area, and the two cells can end up with inconsistent supplies or placement.
- Timing gets harder too, because both delays land on a path that already crosses voltages.
Follow-up Question & Model Response
"If the library has both isolation cells and ELS cells, which one does ICC2 pick for an isolation strategy?"
Candidate Model Response: With no mapping command, ICC2 looks for isolation cells first and falls back to an ELS only if no isolation cell matches. If you map both, it still prefers the isolation cell. If you map only ELS cells, it uses those. The ELS used this way should be modelled as a level shifter followed by an isolation cell.
Practical Example
Design Scenario: (illustrative) PD_SENSOR runs at 0.8 V, can shut down, and drives irq into the 1.0 V always-on domain. One ELS_LH_X1 sits on irq with VDDL at 0.8 V, VDDH at 1.0 V and EN from the controller. When PD_SENSOR is on, irq rises from 0.8 V to 1.0 V swing. When PD_SENSOR is off, EN goes low and irq is held at 0. If the isolation strategy is mapped with use_interface_cell (UPF) to ELS_LH_X1 only, ICC2 uses the ELS even where plain isolation cells exist.
Low-Power & UPF Handbook
Master Low-Power VLSI & Multivoltage Design
Read the complete low-power guide library covering power domains, level shifters, isolation clamps, state retention, and UPF signoff verification.
Offline PDF Bundle
Want all 1109 questions offline?
Get the complete 4-book PDF bundle (PnR, STA, MMMC, Low Power) with a clickable table of contents - no ads, no internet needed.
Continue practising