What is the difference between a low-to-high and a high-to-low voltage crossing?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A low-to-high (LH) crossing occurs when a signal travels from a lower-voltage domain into a higher-voltage domain; a high-to-low (HL) crossing occurs when a signal travels from a higher-voltage domain into a lower-voltage domain.
Mentor Explanation
A worked topology makes this concrete: a 1.0V top domain contains an ARM926 block at 1.2V, a DMA block at 1.1V, and an APB block at 0.9V. Signals leaving the 1.2V block into the 1.0V surround need an HLtype level shifter; signals going the other direction, into the higher-voltage block, need an LH-type shifter. This is mirrored in the '-rule' option of set_level_shifter, which explicitly takes LOCATION: SELF Domain @ 1.0V LS shifter placed inside LOCATION: PARENT Domain @ 1.0V out LS shifter placed outside low_to_high, high_to_low, or both, and a matched pair of strategies is typically written for the two directions of a single domain boundary: one rule high_to_low on inputs, one rule low_to_high on outputs.
Example
In the worked topology PD_TOP (1.1V) containing PD_A (0.9V), the tool inserts LS_H2L at PD_A's input boundary (1.1V-to-0.9V, high-tolow) and LS_L2H at its output boundary (0.9V-to-1.1V, low-to-high).
Why It Matters
I want candidates to see that direction is defined by the signal's path, not by which domain is 'main' — the same domain boundary typically needs both types of shifters, one per direction.
Command
set_level_shifter LS_H2L -domain PD_A -applies_to inputs -rule
high_to_low -location self
set_level_shifter LS_L2H -domain PD_A -applies_to outputs -rule
low_to_high -location parentCommon Beginner Mistake
Forgetting that both directions across the same boundary usually need separate shifter strategies — inserting only one direction's shifter leaves the other direction with an unresolved voltage mismatch.
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