This example illustrates usage of light sleep mode. Unlike deep sleep mode, light sleep preserves the state of the memory, CPU, and peripherals. Execution of code on both CPUs is stopped when `esp_light_sleep_start()` function is called. When the chip exits light sleep mode, execution continues at the point where it was stopped, and `esp_light_sleep_start()` function returns.
This example can be used with any ESP32 development board. Most boards have a button attached to GPIO0, often labelled `BOOT`. If the board does not have such button, an external button can be connected, along with a 10k pull-up resistor, and a 100nF capacitor to ground for debouncing.
If do nothing to the example, the chip will wake-up every 2000 ms by timer, and fall into light sleep again after print some logs. We can see the wake-up reason is `timer` in this case.
### Wake-up by GPIO
For this example, the wake-up GPIO is bound to the 'BOOT' button on the board, we can wake-up the chip from light sleep by pressing the 'BOOT' button, the chip will wake-up immediately after we pressed the button. We can see the wake-up reason is `pin` in this case and the chip will fall into light sleep again if we release the button.
### Wake-up by UART
For this example, the wake-up UART is bound to the default console port (UART_NUM_0), we can wake-up the chip from light sleep by inputting some keys on the key board. We can see the wake-up reason is `uart` in this case.
Note #1: the UART wake-up threshould is set to 3 in this example, which means the ascii code of the input character should has at least 3 edges, for example, the ascii code of character `0` is `0011 0000` in binary, which only contains 2 edges, so the character `0` is not enough to wakeup the chip.
Note #2: only UART0 and UART1 (if has) are supported to be configured as wake up source. And for ESP32, we have to use iomux pin for RX signal (i.e. GPIO3 for UART0 & GPIO9 for UART1), otherwise it won't success.
Note #3: due to limitation of the HW, the bytes that received during light sleep is only used for waking up, and it will not be received by UART peripheral or passed to the driver.
In the scenario above, the button attached to GPIO0 was pressed and held for about 3 seconds, after the 2nd wakeup from light sleep. The program has indicated the wakeup reason after each sleep iteration.
## Current Consumption
In this example, current consumption in light sleep mode is in the range of 0.8 — 1.1 mA. Current consumption in active mode is 28 — 32 mA. Average current consumption is 1.1 - 1.3 mA.
## Troubleshooting
If pressing the button attached to GPIO0 does not affect program behavior, check DTR/RTS configuration in the serial monitor. This is not necessary for IDF monitor, but for other tools it might be necessary to set DTR and RTS line state to "disabled" or "de-asserted".
