ULTRA_LOW_POWER (SystemSleepMode)

sleep() [ Sleep ]

// EXAMPLE
SystemSleepConfiguration config;
config.mode(SystemSleepMode::ULTRA_LOW_POWER)
      .gpio(D2, FALLING);
System.sleep(config);

Since 2.0.0:

The SystemSleepMode::ULTRA_LOW_POWER mode is similar to STOP mode however internal peripherals such as GPIO, UART, ADC, and DAC are turned off. Like STOP mode, the RTC continues to run but since many more peripherals are disabled, the current used is closer to HIBERNATE. It is available in Device OS 2.0.0 and later.

Wake Mode	Gen 2	Gen 3	Gen 4
GPIO	✓	✓	✓
Time (RTC)	✓	✓	✓
Analog		✓
Serial		✓
BLE		✓
Cellular		✓	✓
Wi-Fi		✓

Typical power consumption in ultra-low power (ULP) sleep mode, based on the wakeup source:

Device	GPIO	RTC	Analog	Serial	BLE	Network
P2	579 uA	572 uA	n/a	n/a	n/a	n/a
M404	547 uA	515 uA	n/a	n/a	n/a	n/a
M524	421 uA	415 uA	n/a	n/a	n/a	n/a
T523 Eval	139 uA	139 uA	140 uA	564 uA	214 uA	21.7 mA
T402 Eval	114 uA	114 uA	117 uA	530 uA	186 uA	16.9 mA
Boron 2G/3G	171 uA	174 uA	178 uA	610 uA	494 uA	16.4 mA
Boron LTE	127 uA	128 uA	130 uA	584 uA	442 uA	14.2 mA
B402 SoM	48 uA	47 uA	48 uA	557 uA	130 uA	9.5 mA
B523 SoM	54 uA	55 uA	56 uA	537 uA	139 uA	22.8 mA
Argon	82 uA	81 uA	82 uA	520 uA	141 uA	21.3 mA
Electron	2.42 mA	2.55 mA	n/a	n/a	n/a	n/a
Photon	2.76 mA	2.83 mA	n/a	n/a	n/a	n/a

In this mode:

Real-time clock (RTC) is kept running.
Network is optionally kept running for cellular, similar to SLEEP_NETWORK_STANDBY.
On the Argon, network can optionally be kept running for Wi-Fi.
BLE is kept on if used as a wake-up source (Gen 3 devices only).
UART and ADC are only kept on if used as a wake-up source (Gen 2 and Gen 3 only)
GPIO are kept on; OUTPUT pins retain their HIGH or LOW voltage level during sleep.
Can wake from: Time, GPIO. On some devices: analog, serial, BLE, cellular, and Wi-Fi.
At least one wake source must be specified in ULP sleep mode.
On wake, execution continues after the the System.sleep() command with all local and global variables intact.

Cellular Devices (B-Series SoM, Tracker SoM, Tracker One, Boron, E404X, E-Series, and Electron):

On Gen 3 cellular devices, wake-on network can be enabled in ultra-low power mode. This is recommended for any sleep duration of less than 10 minutes as it keeps the modem active while in sleep mode.
You should avoid powering off and on the cellular modem in periods of less than 10 minutes. Since the cellular modem needs to reconnect to the cellular network on wake, your mobile carrier may ban your SIM card from the network for aggressive reconnection if you reconnect more than approximately 6 times per hour.
The M-SoM does not support wake on network for cellular or Wi-Fi networks.