Power Domain Gating

Power domain gating allows power saving by reducing the voltage in logic areas in the device to a low-power supply. The feature is available in Standby sleep mode and will reduce the voltage in domains where all peripherals are idle. Internal logic will maintain its content, meaning the corresponding peripherals will not need to be reconfigured when normal operating voltage is returned. Most power domains can be in the following three states:

Active state: The power domain is powered on.
Retention state: The main voltage supply for the power domain is switched off, while maintaining a secondary low-power supply for the sequential cells. The logic context is restored when waking up.
Off state: The power domain is entirely powered off. The logic context is lost.

The SAM L21 device contains three power domains which can be controlled using power domain gating, namely PD0, PD1, and PD2. These power domains can be configured to the following cases:

Default with no sleepwalking peripherals: A power domain is automatically set to retention state in standby sleep mode if no activity require it. The application can force all power domains to remain in active state during standby sleep mode in order to accelerate wakeup time.
Default with sleepwalking peripherals: If one or more peripherals are enabled to perform sleepwalking tasks in standby sleep mode, the corresponding power domain (PDn) remains in active state as well as all inferior power domains (<PDn).
Sleepwalking with dynamic power domain gating: During standby sleep mode, a power domain (PDn) in active can wake up a superior power domain (>PDn) in order to perform a sleepwalking task. The superior power domain is then automatically set to active state. At the end of the sleepwalking task, the device can either be woken up or the superior power domain can return to retention state.

Power domains can be linked to each other, it allows a power domain (PDn) to be kept in active state if the inferior power domain (PDn-1) is in active state too.

Table 1 illustrates the four cases to consider in standby mode.

Table 1. Sleep Mode versus Power Domain State Overview
Sleep mode	PD0	PD1	PD2	PDTOP	PDBACKUP
Idle	active	active	active	active	active
Standby - Case 1	active	active	active	active	active
Standby - Case 2	active	active	retention	active	active
Standby - Case 3	active	retention	retention	active	active
Standby - Case 4	retention	retention	retention	active	active
Backup	off	off	off	off	active
Off	off	off	off	off	off