(See the README.md file in the upper level 'examples' directory for more information about examples.)
The [Deep-sleep wake stub](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/deep-sleep-stub.html) is used to RTC fast boot mode that avoid the SPI flash booting, thus speeding up the wakeup process. This example demonstrates how to implement the wake stub.
In this example, the `CONFIG_BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP` Kconfig option is used, which allows you to reduce the boot time of the bootloader during waking up from deep sleep. The bootloader stores in rtc memory the address of a running partition and uses it when it wakes up. This example allows you to skip all image checks and speed up the boot.
This example runs on any commonly available development board with a chip listed under "Supported Targets" without requiring any extra hardware if only **Timer** wake up sources are used.
* **Wake up time** can be configured via `Example configuration > Wake up interval in seconds`
Wake up sources that are unused or unconnected should be disabled in configuration to prevent inadvertent triggering of wake up as a result of floating pins.
### Build and Flash
Build the project and flash it to the board, then run monitor tool to view serial output:
```
idf.py -p PORT flash monitor
```
(Replace PORT with the name of the serial port to use.)
(To exit the serial monitor, type ``Ctrl-]``.)
See the Getting Started Guide for full steps to configure and use ESP-IDF to build projects.
## Example Output
On initial startup, this example will detect that this is the first boot and output the following log:
```
...
I (309) cpu_start: Starting scheduler on PRO CPU.
I (0) cpu_start: Starting scheduler on APP CPU.
Not a deep sleep reset
Enabling timer wakeup, 10s
Entering deep sleep
```
The ESP chips will then enter deep sleep. When a timer wake up occurs, if deep sleep wake stub enabled, the ESP chips will boot from RTC memory and execute stub code. The output log such as the following:
> 1. The wakeup time cost printed in this example is not entirely accurate. This is because it does not take into account the hardware initialization time before the CPU starts. For most ESP chips, the initialization time is about 280 us.
> 2. The wake-up time shown in this example may be reduced by disabling the logs printed by ROM code. For most ESP chips, this ROM printing takes about 6100 us. In the product firmware, users can temporarily or permanently turn off ROM printing by calling ``esp_deep_sleep_disable_rom_logging`` or by setting ``menuconfig`` > ``Boot ROM Behavior`` > ``Permanently disable logging`` to speed up the wake-up.(ESP32 does not support suppressing ROM logging through menuconfig, but it can be suppressed by grounding GPIO15)
> 3. Here is a method for roughly estimating optimal wake-up time: Taking ESP32-C6 as an example, the wake-up time from stub printing is about 6500 us. However, by substracting the ROM printing overhead of 6100 us and adding the system initialization overhead of 280 us, the wake-up overhead is estimated to be around 680 us. Users also can modify the example to configure GPIO wake-up and obtain a more realistic and accurate wake-up time by grabbing GPIO signals with a logic analyzer.
