RTC-Only and RTC+DDR Mode Overview

AM437x supports two RTC modes, RTC+DDR and RTC-Only mode. RTC+DDR Mode is similar to the Suspend/Resume above but only supports wake by the Power Button present on the board or from an RTC ALARM2 Event. RTC-Only mode supports the same wake sources, however DDR context is not maintained so a wake event causes a cold boot.

RTC-Only mode is supported on:

  • AM437x GP EVM
  • AM437x SK EVM

RTC+DDR mode is supported on:

  • AM437x GP EVM


The first step in using RTC+DDR mode is to enable off mode by typing the following at the command line:

$ echo 1 > /sys/kernel/debug/pm_debug/enable_off_mode

With off-mode enabled, a command to enter DeepSleep0 will now enter RTC-Only mode:

$ echo mem > /sys/power/state

this method of entry only supports Power button as the wake source.

To use the rtc as a wake source, after enabling off mode use the following command:

$ rtcwake -s <NUMBER OF SECONDS TO SLEEP> -d /dev/rtc0 -m mem

Whether or not your board enters RTC-Only mode or RTC+DDR mode depends on the regulator configuration and whether or not the regulator that supplies the DDR is configured to remain on during suspend. This is supported by the TPS65218 in use of the AM437x boards but not the TPS65217 or TPS65910 present on AM335x boards.

tps65218: tps65218@24 {
        reg = <0x24>;
        compatible = "ti,tps65218";
        interrupts = <GIC_SPI 7 IRQ_TYPE_NONE>; /* NMIn */
        interrupt-parent = <&gic>;
        #interrupt-cells = <2>;


        dcdc3: regulator-dcdc3 {
                compatible = "ti,tps65218-dcdc3";
                regulator-name = "vdcdc3";
                regulator-min-microvolt = <1500000>;
                regulator-max-microvolt = <1500000>;



Another important thing to make sure of is that you are using the proper u-boot. A certain u-boot is required in order to support RTC+DDR mode otherwise the following message appears during boot of the kernel:

PM: bootloader does not support rtc-only!

When building u-boot, rather than using am43xx_evm_config you must use am43xx_evm_rtconly_config to support either RTC mode.

RTC-Only Mode

RTC-Only mode does not maintain DDR context so placing a board into RTC-only mode allows for very low power consumption after which a supported wake source will cause a cold boot. RTC-Only mode is entered via the poweroff command.

To wakeup from RTC-Only mode via an RTC alarm, a separate tool must be used to program an RTC alarm prior to entering poweroff.