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ESP32 deep sleep does not pause and later resume your application. It powers down the CPUs, most RAM, and APB-clocked digital peripherals. After a wake event, the chip starts through its boot path again. Only state deliberately kept in powered RTC memory can persist across that restart.
What stays on during ESP32 deep sleep?
For the ESP32 covered by Espressif’s ESP-IDF guide, the powered domains listed for deep sleep are the RTC controller, ULP coprocessor, RTC FAST memory, and RTC SLOW memory. The CPUs, most RAM, and digital peripherals clocked from APB are powered off. Wireless connections are not maintained. Espressif notes that RTC domains not needed by the selected wake sources are generally powered down by default, though an application can configure power behavior. See Espressif’s ESP32 sleep-mode guide.
RTC memory can retain deliberately saved state
Data placed in RTC memory can survive a deep-sleep wake if its memory domain remains powered. Espressif places variables marked with RTC_DATA_ATTR in RTC SLOW memory by default and keeps that domain powered by default when such variables are used. Power configuration can override that behavior, so retention depends on both where data is stored and whether the relevant RTC domain stays on.
Ordinary variables in RAM that is powered off should be treated as fresh application state after wake. If the device needs to remember a reading, counter, or other value, store it in retained RTC memory or use another appropriate persistence method rather than assuming the old task state will still exist.
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Does the ESP32 resume where it left off?
No. Deep-sleep wake starts the application again through a boot path; it does not restore the former task stack or ordinary CPU state. Most peripherals are in reset state. This differs from light sleep, where CPUs, digital peripherals, and most RAM resume with state preserved.
An optional wake stub can execute before normal initialization. It must reside in RTC FAST memory, and at that early stage it can call only ROM code or code also located in RTC FAST memory. Espressif describes a partial initialization before the stub runs: RTC FAST memory is validated, most peripherals remain reset, and SPI flash has not yet been mapped. See Espressif’s deep-sleep wake-stub guide.
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What can wake an ESP32 from deep sleep?
Espressif’s ESP32 guide documents several wake routes. They can be combined; the chip wakes when a configured source triggers. A wake source may remain configured after a wake, so disable it if it should not apply to a later sleep cycle.
| Wake source | What triggers wake | Practical requirements and limits |
|---|---|---|
| RTC timer | A configured time interval elapses. | The API takes microseconds, but actual timing resolution depends on the RTC slow-clock source. |
| Touch sensor | A configured touch-pad interrupt occurs. | Configure the touch-pad interrupt before entering deep sleep. |
| ext0 | One RTC GPIO reaches a selected logic level. | Keeps the RTC peripheral domain on. Configure RTC GPIO pull-up or pull-down as needed; after wake, call rtc_gpio_deinit() before using that pad as a digital GPIO. |
| ext1 | Selected RTC GPIOs meet a chosen condition: any selected pin high, or all selected pins low. | Internal pulls require RTC peripheral power or suitable hold behavior; otherwise external resistors may be needed. |
| ULP coprocessor | A program running while the main CPU sleeps detects a programmed condition, such as a sensor or ADC/GPIO change. | The ULP program runs from RTC SLOW memory and can wake the main CPU when its condition is met. |
| Deep-sleep GPIO wake | A supported GPIO condition occurs on a pin powered by VDD3P3_RTC. | Supported pins depend on the exact chip and target datasheet; do not assume every GPIO or ESP32-family chip supports the same mechanism. |
Choose a trigger based on the signal and wiring
- Use the RTC timer when the board should wake on a schedule rather than wait for an external event.
- Use ext0 for a single RTC GPIO level, or ext1 when a condition across multiple RTC GPIOs is needed.
- Use touch wake for a configured touch-pad event, or the ULP when a condition must be checked while the main CPU is off.
- Check the target chip’s documentation before wiring a deep-sleep wake signal. Valid pins and capabilities are chip-specific; the family name alone does not establish compatibility.
How do I tell what woke the ESP32?
Call esp_sleep_get_wakeup_cause() after wake to obtain a wake-source cause. Espressif warns that this API reports only one source, so if multiple sources trigger at the same time, the returned cause may not preserve all trigger information.
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For ext1 wake, use esp_sleep_get_ext1_wakeup_status() to identify the GPIOs reported as wake inputs. Touch wake has its own status API. If your application combines sources and needs to know precisely which input changed, use the source-specific status information where available rather than relying only on the single general cause.
Deep sleep or light sleep?
Choose deep sleep when the device can restart its application after waking and you can preserve any required state deliberately. Choose light sleep when the application needs CPU, peripheral, and most RAM state preserved across the sleep interval. In either case, confirm that the specific wake source and pin are supported by your target ESP32 chip.
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