ExpertQuestion 4 of 50

What does a TOP-level power-intent structure look like, using the Mychip example as a reference?

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

A top-level UPF runs in a fixed order: domains, then supply ports and nets, then each domain primary supply, then switches and strategies, and last the power states. In MYCHIP that means four create_power_domain (UPF) calls, VDD1p0, VDD0p9, VDDdsp and VSS shared into subdomains with -reuse, a switch that makes VDD1p0_SW for PD_COP, and a table of legal rail combinations. Each section may only refer to objects an earlier section created.

Flow of UPF file sections in order: 1 create_power_domain for PD_MYCHIP, PD_CPU, PD_DSP, PD_COP; 2 supply ports and nets VDD1p0, VDD0p9, VDDdsp, VSS with -reuse; 3 set_domain_supply_net (PD_COP on VDD1p0_SW); 4 create_power_switch COP_SW; 5 isolation, retention, level shifters; 6 create_pst and add_pst_state, all tagged (UPF). — Technical Reference DiagramWhat does a TOP-level power-intent structure look like, using the Mychip example as a reference?

Technical Reference DiagramWhat does a TOP-level power-intent structure look like, using the Mychip example as a reference?

Technical Explanation

Domains first: PD_MYCHIP at the top, then PD_CPU, PD_DSP and PD_COP by instance. No later command can name a domain that does not exist yet.
Supply nets: create each rail in PD_MYCHIP, then extend it with create_supply_net -reuse (UPF) so a subdomain shares that net instead of creating it again.
Primary supplies: set_domain_supply_net (UPF) gives each domain one primary power and one primary ground; PD_COP gets VDD1p0_SW.
Switch: create_power_switch (UPF) turns VDD1p0 into VDD1p0_SW under control of PSE from U_PC.
Strategies: isolation and retention for PD_COP run from VDD1p0; level shifters cover the VDD0p9 and VDDdsp crossings.
States last: create_pst (UPF) and add_pst_state (UPF) list the legal rail combinations, and any combination left out is illegal.
This page keeps the legacy form many flows still run. The 1801-2015 form uses create_power_domain -supply {primary SS_COP} (UPF) and add_power_state (UPF).

# [UPF]  mychip.upf
create_power_domain PD_MYCHIP -include_scope
create_power_domain PD_COP -elements {U_COP}
create_supply_net VDD1p0 -domain PD_MYCHIP
create_supply_net VDD1p0 -domain PD_COP -reuse
create_supply_net VSS -domain PD_MYCHIP
create_supply_net VSS -domain PD_COP -reuse
create_supply_net VDD1p0_SW -domain PD_COP
set_domain_supply_net PD_MYCHIP -primary_power_net VDD1p0 -primary_ground_net VSS
set_domain_supply_net PD_COP -primary_power_net VDD1p0_SW -primary_ground_net VSS
create_power_switch COP_SW -domain PD_COP -input_supply_port {in VDD1p0} -output_supply_port {out VDD1p0_SW} -control_port {ctrl PSE} -on_state {ON in {ctrl}}
create_pst MYCHIP_PST -supplies {VDD1p0 VDD0p9 VDDdsp VDD1p0_SW}

What To Check

Every domain has exactly one primary power and one primary ground before commit_upf (ICC2).
Shared rails are one net extended with -reuse, not created a second time.
PD_COP primary is VDD1p0_SW, and the COP_SW input is VDD1p0.
The table lists every rail and every mode the chip really enters.

Command Checks & Actions

UPF (design.upf)set_domain_supply_net PD_COP -primary_power_net VDD1p0_SW -primary_ground_net VSS

Sets the switched rail and ground as PD_COP primary supplies

UPF (design.upf)create_pst MYCHIP_PST -supplies {VDD1p0 VDD0p9 VDDdsp VDD1p0_SW}

Declares the table columns, one per rail

ICC2 (icc2_shell)load_upf mychip.upf

Reads the file; a section out of order stops the load at the first unknown object

ICC2 (icc2_shell)report_power_domains

Confirms each domain primary supply

ICC2 (icc2_shell)report_pst -derived

Lists the merged state table the tool derived

Healthy, Suspicious & Hard-stop Results

Healthy (illustrative): report_power_domains (ICC2) shows PD_COP on VDD1p0_SW and the other three on always-on rails, and report_pst (ICC2) lists every planned mode.
Suspicious (illustrative): report_pst (ICC2) shows a mode nobody planned, or a subdomain net that is not the top VDD1p0.
Hard stop: A domain has no primary ground, or PD_COP never gets the top VDD1p0 because -reuse was dropped.

Common Mistake

The Trap: Dropping the dash so the line reads primary_ground_net VSS instead of -primary_ground_net VSS.

The command errors out, PD_COP gets no primary ground, and every later check that needs that domain supply fails with it.

What The Interviewer Is Testing

Whether you know the dependency order inside a UPF file.
Whether you can read and fix legacy UPF and name its 1801-2015 replacement.

Follow-up Question & Model Response

"Why extend a net with -reuse instead of creating it again in the subdomain?"

Candidate Model Response: With -reuse, create_supply_net does not create a net; it makes the existing VDD1p0, created for PD_MYCHIP, available in the extent of PD_COP. Without it the command tries to create VDD1p0 a second time instead of sharing the top rail. The standard requires -domain on both the creating and the reusing command. The -resolve value of the reusing command must not conflict with the creating one.

Practical Example

Design Scenario: (illustrative) Someone moves the set_domain_supply_net (UPF) line for PD_COP above create_supply_net VDD1p0_SW -domain PD_COP (UPF). VDD1p0_SW is unknown when that line runs, so the load stops there. Put back in order, the file loads, report_pst (ICC2) shows four modes PM1 to PM4, and check_mv_design (ICC2) moves on to the crossings.

Low-Power & UPF Handbook