esp-idf/examples/peripherals/uart/uart_select/README.md

Supported Targets	ESP32	ESP32-C2	ESP32-C3	ESP32-C5	ESP32-C6	ESP32-C61	ESP32-H2	ESP32-P4	ESP32-S2	ESP32-S3

UART Select Example

(See the README.md file in the upper level 'examples' directory for more information about examples.)

The UART select example is for demonstrating the use of select() for synchronous I/O multiplexing on the UART interface. The example waits for a character from UART using select() until a blocking read without delay or a successful non-blocking read is possible.

Please note that the same result can be achieved by using uart_read_bytes() but the use of select() allows to use it together with other virtual file system (VFS) drivers, e.g. LWIP sockets.

This example can be used to develop applications for non-blocking read and write from/to various sources (UART, sockets, ...) where a ready resource can be served without being blocked by a busy resource.

For a more comprehensive example please refer to system/select.

How to use example

Hardware Required

The example can be run on any development board, that is based on the Espressif SoC. The board shall be connected to a computer with a single USB cable for communication through UART.

Configure the project

idf.py menuconfig

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

(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

You can see a similar output after flashing and monitoring the application:

...
I (276) cpu_start: Pro cpu start user code
I (294) cpu_start: Starting scheduler on PRO CPU.
I (0) cpu_start: Starting scheduler on APP CPU.
I (10295) uart_select_example: Timeout has been reached and nothing has been received
I (15295) uart_select_example: Timeout has been reached and nothing has been received
I (20295) uart_select_example: Timeout has been reached and nothing has been received

You can push any key on your keyboard to see the result of a successful detection by select() and subsequent blocking read (without delay). The following output shows the result of pushing a on the keyboard:

...
I (15295) uart_select_example: Timeout has been reached and nothing has been received
I (20295) uart_select_example: Timeout has been reached and nothing has been received
I (20945) uart_select_example: Received: a
I (25955) uart_select_example: Timeout has been reached and nothing has been received
...