3.3.4. Low Power Modes

3.3.4.1. Overview

The following sections describe a high-level description of the different low power modes (LPM) of the device. If your application requires inactive power management, you must determine which low power mode described below satisfies your requirements. Each mode must be evaluated based on power consumption and latency (the time it takes to wakeup to active mode) requirements. Specific values are detailed in the device-specific data sheet. As part of this SDK offering, Texas Instruments has added support for the following low power modes (ordered from lowest power consumption to highest power consumption):

RTC Only
RTC Only Plus DDR
Deep Sleep

3.3.4.2. RTC Only

RTC Only mode is the deepest low power mode that allows the system to enter a complete poweroff state with ultra-low power consumption while maintaining system time and wakeup capability. Unlike other low power modes, RTC Only powers off all domains except the RTC, which remains active to keep track of system time and provide wakeup via RTC alarm or RTC I/O event.

Note

The system loses nearly all its state as DDR is also turned-off. RTC Only is comparable with a Linux poweroff state with system timer ON.

Important

Jumper J14 position on the EVM determines which low power mode is entered. For RTC Only mode, connect jumper J14 to the position marked as “RTC ONLY MODE”.

The reference implementation in this SDK implements RTC Only as a poweroff state. When the system powers off, the RTC driver programs the RTC hardware for poweroff mode. The final sequence to power off the system is executed by TF-A firmware, which pulls the PMIC_EN signal low to turn off all the supply rails powered by the PMIC.

On AM62L platforms, if the system-power-controller device tree property is not set on the PMIC node, the PMIC will not register a poweroff handler. In this configuration, TF-A firmware handles the poweroff sequence and the system enters RTC Only mode. This allows the system to maintain accurate system time and provide wakeup capability via RTC alarm or external button press while consuming minimal power.

RTC Only mode supports two wakeup sources: RTC timer alarm and RTC I/O pins.

Use the following command to enter RTC Only mode with a timer alarm for wakeup:

root@<machine>:~# rtcwake -s <time> -m off

Use the following command to enter RTC Only mode with RTC I/O (button press) for wakeup:

root@<machine>:~# poweroff

At this point, the Linux kernel will go through its poweroff process and the console output will stop at the following lines:

[   51.698039] systemd-shutdown[1]: Powering off.
[   51.769478] reboot: Power down

The system has entered RTC Only mode and can be woken up by activity on an RTC I/O pin programmed for wakeup. If an alarm was set, the system will automatically wake up when the alarm triggers.

During resume from RTC Only mode, the system goes through a normal Linux boot process. The RTC driver detects that the RTC is already programmed and skips the full initialization, performing only minimal cleanup to preserve the system time.

3.3.4.3. RTC Only Plus DDR

Note

Please go through the s2idle docs to understand how to select between multiple low power modes. The steps and overall architecture/ sequence diagrams are documented in Suspend-to-Idle (S2Idle) and PSCI Integration. The default mode via s2idle is RTC Only Plus DDR, since it’s the deepest.

If regular [mem] interface is selected in /sys/power/mem_sleep, the RTC Only + DDR low power mode requires special steps to enter. The steps are documented in System Suspend Mode Workarounds.

RTC Only + DDR mode is the deepest low power mode that allows the system to enter a state of lowest power consumption while still retaining the DDR RAM context.

Important

Jumper J14 position on the EVM determines which low power mode is entered. For RTC Only + DDR mode, connect jumper J14 to the position marked as “RTC + DDR MODE” (different from RTC Only Mode).

In order to enter RTC Only + DDR mode, first disable USB0 and USB1 as wakeup sources.

root@am62lxx-evm:~# echo disabled > /sys/devices/platform/bus@f0000/f900000.dwc3-usb/power/wakeup
root@am62lxx-evm:~# echo disabled > /sys/devices/platform/bus@f0000/f910000.dwc3-usb/power/wakeup

Now the SoC can be suspended using the following command.

root@am62lxx-evm:~# rtcwake -s 5 -m mem
rtcwake: assuming RTC uses UTC ...
rtcwake: wakeup from "mem" using /dev/rtc0 at Thu Jan  1 00:04:20 1970
[  222.269447] PM: suspend entry (s2idle)
[  222.273481] Filesystems sync: 0.000 seconds
[  222.281882] Freezing user space processes
[  222.291800] Freezing user space processes completed (elapsed 0.002 seconds)
[  222.298873] OOM killer disabled.
[  222.302177] Freezing remaining freezable tasks
[  222.308071] Freezing remaining freezable tasks completed (elapsed 0.001 seconds)
[  222.349890] xhci-hcd xhci-hcd.1.auto: xHC error in resume, USBSTS 0x401, Reinit
[  222.357227] usb usb1: root hub lost power or was reset
[  222.362980] xhci-hcd xhci-hcd.1.auto: USB3 root hub has no ports
[  222.395078] OOM killer enabled.
[  222.398356] Restarting tasks: Starting
[  222.419490] Restarting tasks: Done
[  222.430220] random: crng reseeded on system resumption
[  222.445468] PM: suspend exit

3.3.4.4. DeepSleep

DeepSleep AKA Suspend-to-RAM is a low-power mode that allows the SoC to retain its state in RAM while the processor is turned off. DeepSleep mode is typically used during inactivity when the user requires very low power while waiting for an event that requires processing or higher performance. DeepSleep is a low power mode which still includes DDR in self-refresh, so wakeup events do not require a full cold boot, significantly reducing wakeup latencies. Since the power to Always-On power domains are ON throughout DeepSleep, power to key modules such as GPIO and others is maintained to allow wakeup events to exit out of this mode.

In order to enter DeepSleep,

root@am62lxx-evm:~# echo mem > /sys/power/state
[   88.507502] PM: suspend entry (deep)
[   88.530523] Filesystems sync: 0.019 seconds
[   88.537597] Freezing user space processes
[   88.543218] Freezing user space processes completed (elapsed 0.001 seconds)
[   88.550275] OOM killer disabled.
[   88.553518] Freezing remaining freezable tasks
[   88.559139] Freezing remaining freezable tasks completed (elapsed 0.001 seconds)
[   88.566570] printk: Suspending console(s) (use no_console_suspend to debug)
ERROR:   Wake up src 0x80
ERROR:   Wake up interrupt 0xc
[   88.591181] Disabling non-boot CPUs ...
[   88.593309] psci: CPU1 killed (polled 0 ms)
[   88.594858] Enabling non-boot CPUs ...
[   88.595327] Detected VIPT I-cache on CPU1
[   88.595379] GICv3: CPU1: found redistributor 1 region 0:0x0000000001860000
[   88.595446] CPU1: Booted secondary processor 0x0000000001 [0x410fd034]
[   88.596420] CPU1 is up
[   88.634778] OOM killer enabled.
[   88.637926] Restarting tasks ... done.
[   88.642801] random: crng reseeded on system resumption
[   88.649913] PM: suspend exit
root@am62lxx-evm:~#

3.3.4.5. Memory Usage

The following table summarizes the usage of memory in different modes of operation of the device.

Domain	Memory	Boot Operation	Normal Operation	Deep Sleep	RTC Only + DDR
WKUP	TIFS SRAM (196 KB)	TIFS load (144 KB) + Sec ROM (20 KB)	TIFS (144 KB)	TIFS (144 KB)	TIFS (144 KB) SEC ROM (20 KB) TIFS_STUB (32 KB)
WKUP	WKUP PSRAM (512 KB)	Pub ROM (64 KB) or (exclusively) PreBL Stack & runtime data (64 KB)		A53 Stub, TF-A Stub (64 KB)	Pub ROM (64 KB)
Main	MAIN MSRAM (96 KB)	PreBL (64 KB) DDR initialization Pub ROM (8 KB) ROM IPC (8 KB)	TIFS IPC (24 KB)	TIFS IPC (24 KB)	PreBL (64 KB) Non-destructive DDR initializtion TIFS IPC + ROM IPC (24 KB) Pub ROM (8 KB)
Main	DDR	Linux TF-A	Linux TF-A	Linux TF-A	Linux TF-A