Module 71 min
Retention Registers
When a power domain is switched off, all register state is lost. Retention registers have a small always-on "shadow latch" that preserves the register value thr
When a power domain is switched off, all register state is lost. Retention registers have a small always-on "shadow latch" that preserves the register value through power-off, then restores it when the domain powers back on.
Save / Restore Sequence
Power down:
- Assert SAVE signal → shadow latch captures the main FF value.
- Assert isolation → outputs clamped.
- Cut power switch → domain powers off (shadow latch stays on AON power).
Power up:
- Enable power switch → domain powers on.
- Assert RESTORE signal → main FF restores from the shadow latch.
- Deassert isolation → normal operation resumes.
Retention Register Tradeoffs
Area cost: ~2–3× larger than regular FF Leakage: Shadow latch draws small AON leakage Not all registers need retention - only state that's expensive to reconstruct (cache tags, config registers). Data that can be reloaded from memory doesn't need retention. UPF command: set_retention PD_CPU -save_signal {save 1} -restore_signal {restore 1}