What is the difference between a low-to-high and a high-to-low voltage crossing?

Short Answer

A low-to-high crossing goes from a lower-voltage domain into a higher one, and a high-to-low crossing goes the other way. Low-to-high needs a level shifter, because the weak input cannot turn off the receiver PMOS. High-to-low often works without one, so it depends on the library and the threshold you set.

Technical Explanation

• Direction comes from the signal path, not from which domain is "main": one boundary usually has both types.
• Low-to-high needs a shifter, or the receiver leaks crowbar current and switches slowly.
• High-to-low is logically safe in most cases; the library threshold or -threshold decides whether a cell goes in.
• set_level_shifter -rule (UPF) takes low_to_high, high_to_low or both, and -location picks where the cell goes; this page uses self and parent.
• A -rule or -no_shift that blocks insertion turns the missing shifter into a warning, not an error, in check_mv_design (ICC2).
• Cell names LS_L2H and LS_H2L on this page are illustrative.

# [UPF]  design.upf
set_level_shifter LS_H2L -domain PD_GPU -applies_to inputs -rule high_to_low -location self
set_level_shifter LS_L2H -domain PD_GPU -applies_to outputs -rule low_to_high -location parent
# [ICC2]  icc2_shell
check_mv_design
analyze_mv_design -level_shifter -through u_gpu/done

What To Check

• Every PD_GPU output feeding 0.9 V logic has a low-to-high shifter.
• High-to-low inputs match your library threshold decision, with no surprise gaps.
• The voltages in the power states cover the lowest DVFS level, not only nominal.
• Shifter warnings in check_mv_design (ICC2) are explained by a deliberate -rule or -no_shift.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): check_mv_design shows no level shifter errors, and every 0.75 V to 0.9 V output has an LS_L2H.
• Suspicious (illustrative): Warnings on 0.9 V to 0.75 V inputs that nobody has checked against the library threshold.
• Hard stop: Any 0.75 V to 0.9 V crossing left without a shifter, error or warning.

What The Interviewer Is Testing

• That you name the direction from the signal path and know low-to-high is the one that needs a cell.
• That you can say what decides the high-to-low case: library threshold and strategy.
• That you check insertion with a report instead of trusting the UPF.

Follow-up Question & Model Response

"Why would a team still add high-to-low shifters when the logic works without them?"

Candidate Model Response: Some libraries characterise cells only for inputs up to their own supply, so a higher input is outside the timing model. A shifter also gives a clean, characterised delay across the boundary. The library threshold and your -threshold setting make that call per crossing. Check the library notes before you drop them.