IntermediateQuestion 16 of 60

What are always-on control paths and always-on buffers used for in a real design?

From PDVerse Low-Power Physical Design Mentor Guide, part of the pdVerse Mentor Guide

By Tabish Iqbal, who has 9+ years of VLSI Physical Design experience

Short Answer

Always-on control paths carry the signals that manage a shutdown domain, such as isolation enable, save, restore and switch enable, and they must keep working while that domain is off. When such a path runs through a switched voltage area and needs buffering, it needs always-on buffers: cells with a backup supply pin that stays on. ICC2 inserts and legalizes them itself, but only if the library has always-on buffers and the backup supply can reach them.

Floorplan of the hatched PD_COP voltage area with U_PC in the tinted PD_MYCHIP area: SAVE, NRESTORE, ISE and PSE routes enter PD_COP and pass through dual-rail AO buffers whose backup pins tie to a VDD1p0 secondary strap, while one ordinary buffer on VDD1p0_SW sits on the SAVE route drawn in red as the failure case. — Technical Reference DiagramWhat are always-on control paths and always-on buffers used for in a real design?

Technical Reference DiagramWhat are always-on control paths and always-on buffers used for in a real design?

Technical Explanation

Which signals: isolation enable (ISE), SAVE and NRESTORE, switch enable (PSE) and its acknowledge, and clocks or resets for always-on logic inside PD_COP.
Always-on cell: functionally a buffer or inverter, with a backup power pin that stays on during shutdown.
Library need: ICC2 performs always-on optimization only when the target library holds buffers and inverters marked always_on.
Single or dual rail: ICC2 picks dual-rail or single-rail always-on buffers from cell availability and physical constraints.
Legalization: create_mv_cells -always_on (ICC2) runs only always-on legalization of buffers and inverters; no other MV cells are inserted.
Dual-rail buffers need a backup supply strap in the switched area, so plan secondary PG with the voltage area.
An ordinary buffer on VDD1p0_SW in one of these paths cuts the control exactly when PD_COP powers down.

# [ICC2]  icc2_shell
create_mv_cells -always_on
get_cells -filter is_always_on_logic==true -hierarchical
check_mv_design

What To Check

Every control net that crosses PD_COP is driven and buffered from always-on supplies.
Buffers on those nets inside PD_COP are always-on cells with a connected backup pin.
The backup supply has straps wherever always-on buffers can land.
The library contains cells marked always_on.

Command Checks & Actions

ICC2 (icc2_shell)create_mv_cells -always_on

Run always-on legalization of buffers and inverters only

ICC2 (icc2_shell)get_cells -filter is_always_on_logic==true -hierarchical

List the cells ICC2 treats as always-on

ICC2 (icc2_shell)check_mv_design

Report control paths buffered on a supply that turns off

VC LP (vc_static_shell)report_violations -app LP

Review static checks such as ISO_CONTROL_CONN on the enable path

Healthy, Suspicious & Hard-stop Results

Healthy (illustrative): Every buffer on ISE, SAVE, NRESTORE and PSE inside PD_COP is an always-on cell tied to VDD1p0, and check_mv_design is clean.
Suspicious (illustrative): Long always-on routes through PD_COP with many dual-rail buffers, which strain the backup strap.
Hard stop: An ordinary buffer on VDD1p0_SW in the SAVE path, or ISO_CONTROL_CONN from VC LP on the isolation enable.

Common Mistake

The Trap: Letting a timing fix insert an ordinary buffer on VDD1p0_SW into the ISE path inside PD_COP.

When PD_COP turns off the buffer dies, isolation enable floats, and the PD_COP outputs are no longer clamped.

What The Interviewer Is Testing

Can you list which signals need always-on paths and why?
Do you know the tool needs always-on library cells and a backup supply to do this?

Follow-up Question & Model Response

"Why not route every control path around the shutdown area instead?"

Candidate Model Response: Sometimes you can, and it avoids backup straps inside PD_COP. But the targets of these signals, such as isolation cells and retention flops, often sit inside PD_COP, so the path has to enter the area anyway. Long detours also cost timing on the isolation enable, which must switch before the rail drops. Most designs combine short detours with always-on buffers for the last stretch.

Practical Example

Design Scenario: (illustrative) U_PC sits in PD_MYCHIP and drives ISE, SAVE, NRESTORE and PSE into PD_COP. SAVE must reach 300 retention flops spread across a 600 um by 400 um PD_COP area. ICC2 buffers it with dual-rail always-on buffers whose backup pins tie to a VDD1p0 strap every few rows. If one ordinary buffer on VDD1p0_SW slips into that tree, SAVE floats at the flops beyond it for the whole off period, and a spurious save can overwrite their retained state.

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