How do you write a retention strategy with set_retention, and what supplies does it need?

Short Answer

set_retention (UPF) names which registers in a shutdown domain must keep their state, which supply keeps that state alive, and which signals save and restore it. The register needs two supplies: the switched primary for normal operation and a retention supply that stays on while the primary is off. In IEEE 1801-2015 you give that supply as a set with -retention_supply and put the save and restore signals on the same command.

Technical Explanation

• Elements: -elements lists instances or registers; without it, the strategy covers every sequential cell in the domain.
• Retention supply: -retention_supply SS_AON powers the shadow storage and the save and restore control. It must stay on while PD_COP is off.
• Signals: -save_signal {U_PC/SAVE high} -restore_signal {U_PC/NRESTORE low} give each net and its active level or edge.
• Legacy form (still accepted by ICC2/PT): -retention_power_net VDD1p0 -retention_ground_net GND, with signals on set_retention_control (UPF).
• In ICC2, giving only the legacy power net makes the primary ground the retention ground.
• Mapping: map_retention_cell (UPF) limits which library cells may implement the strategy.
• IEEE 1801-2015 makes it an implementation error to leave the retention supply unspecified.

# [UPF]  mychip.upf
set_retention PD_COP_RET -domain PD_COP -retention_supply SS_AON -elements {U_COP/reg1 U_COP/pc U_COP/int_state} -save_signal {U_PC/SAVE high} -restore_signal {U_PC/NRESTORE low}
map_retention_cell PD_COP_RET -domain PD_COP -lib_cells {RET_DFF_X1}
# Legacy form (still accepted by ICC2/PT), signals move to set_retention_control
set_retention PD_COP_RET -domain PD_COP -retention_power_net VDD1p0 -retention_ground_net GND -elements {U_COP/reg1 U_COP/pc U_COP/int_state}
# [ICC2]  icc2_shell
create_mv_cells
report_mv_cells -retention

What To Check

• The retention supply is on in every state where PD_COP is off.
• The element list covers the state software expects after wake-up, and nothing more.
• Save and restore nets come from always-on logic.
• The mapped cells really are retention cells with matching save and restore pins.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): report_mv_cells -retention lists exactly the PD_COP_RET registers, each mapped to a retention cell on SS_AON.
• Suspicious (illustrative): Far more retention flops than expected because -elements was left out, so every PD_COP flop retains.
• Hard stop: The retention supply resolves to SS_COP or VDD1p0_SW, so the saved state dies with the domain.

What The Interviewer Is Testing

• Do you know retention needs a second, always-on supply?
• Can you write the 1801-2015 form and read the legacy one?

Follow-up Question & Model Response

"Why not retain every register in PD_COP?"

Candidate Model Response: Each retention flop is bigger than a plain flop and leaks from the always-on supply for the whole off period. Most state can be rebuilt by reset or reloaded by software faster than you think. Retain only what software cannot cheaply rebuild, such as the program counter and interrupt state. Leaving -elements out retains everything, which is rarely what you want.