An always-on domain is a power domain that remains continuously powered and never shuts down — providing essential infrastructure services (control, monitoring, wake-up logic) that must function even when all other power domains on the chip are in deep sleep or completely powered off.

Why Always-On Domains Exist

• Power gating shuts down blocks to save leakage power — but something must stay awake to:
• Detect wake-up events: Monitor interrupt lines, timers, or external signals that trigger power-up.
• Control power switches: The logic that asserts power switch enables must be powered on to turn other domains back on.
• Generate isolation signals: Isolation cells need control signals from powered logic.
• Maintain retention: Retention flip-flop control signals come from always-on logic.
• Provide clock/reset: Basic clock and reset distribution may need to be always available.

What Lives in the Always-On Domain

• Power Management Unit (PMU): Controls all power switches, isolation cells, retention signals, and power-up/down sequencing.
• Wake-Up Controllers: Monitor wake-up sources (GPIO interrupts, RTC timer, external reset) and initiate the power-up sequence.
• Always-On Timers: Real-time clock (RTC), watchdog timer — must keep running during chip-level sleep.
• Voltage Regulators/PMICs Interface: The interface to external power management ICs.
• I/O Pads: Some I/O pads must remain powered for wake-up signal detection.
• Retention/Isolation Control: Logic that generates SAVE, RESTORE, and ISO signals.

Always-On Domain Design Constraints

• Minimum Logic: Keep the always-on domain as small as possible — every gate in this domain leaks continuously.
• Low-Leakage Cells: Use high-Vth (HVT) standard cells for minimum leakage power.
• Low Voltage: Often operated at the lowest possible voltage to minimize leakage.
• Separate Power Grid: Has its own VDD rail (real VDD, not virtual) — independent of all switchable domains.

Power Architecture

• Switchable Domains: Connected to VDD through power switches → can be turned off.
• Always-On Domain: Connected directly to VDD → always powered.
• Interface: Isolation cells at every boundary between switchable and always-on domains.
• Level Shifters: If always-on domain runs at a different voltage than other domains.

Always-On Domain in UPF

create_power_domain AON -elements {pmu_logic wakeup_ctrl rtc}
create_power_domain CORE -elements {cpu_core} 
  -supply {VDD_sw} -shutoff_condition {pmu_logic/core_sleep}

The always-on domain is defined without a shutoff condition — it has no power switch.

Tradeoff

• The always-on domain represents an irreducible leakage floor — the minimum power the chip consumes even in deepest sleep.
• Minimizing the always-on domain area and leakage is critical for ultra-low-power applications (IoT, wearables, implantable devices).

The always-on domain is the watchkeeper of a power-managed SoC — it stays awake so the rest of the chip can safely sleep, enabling aggressive power gating without losing the ability to wake up.