esp-idf/examples/system/ulp_riscv/uart_print/README.md

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| Supported Targets | ESP32-S2 | ESP32-S3 |
| ----------------- | -------- | -------- |
# ULP-RISC-V bitbanged UART transmit:
This example demonstrates how to program the ULP-RISC-V coprocessor to bitbang an UART TX line. This can be used to log output directly from the ULP, even when the main CPU is in deep sleep.
## How to use example
### Hardware Required
The example can be run on any ESP32-S2 or ESP32-S3 based development board connected to a computer with a single USB cable for flashing and monitoring. The external interface should have 3.3V outputs. You may use e.g. 3.3V compatible USB-to-Serial dongle.
### Setup the Hardware
Connect the external serial interface to the board as follows.
```
-----------------------------------------------------------------------------------------
| Target chip Interface | Kconfig Option | Default ESP Pin | External UART Pin |
| ----------------------|--------------------|----------------------|--------------------
| Transmit Data (TxD) | EXAMPLE_UART_TXD | GPIO4 | RxD |
| Ground | n/a | GND | GND |
-----------------------------------------------------------------------------------------
```
Note: Some GPIOs can not be used with certain chips because they are reserved for internal use. Please refer to UART documentation for selected target.
### Configure the project
Use the command below to configure project using Kconfig menu as showed in the table above. The baudrate can be changed with by configuring `ULP_RISCV_UART_BAUDRATE` under `components` -> `ULP`, but it is recommended to keep the baudrate low, as driver will not be able to accurately output higher rates.
```
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
Output from main CPU:
```
Not a ULP wakeup, initializing it!
Entering in deep sleep
```
Output from ULP-RISCV:
``
Hello World from ULP-RISCV!
Cnt: 0x00000000
Hello World from ULP-RISCV!
Cnt: 0x00000001
Hello World from ULP-RISCV!
Cnt: 0x00000002
Hello World from ULP-RISCV!
Cnt: 0x00000003
Hello World from ULP-RISCV!
Cnt: 0x00000004
``
## Troubleshoot
If the received output contains garbled characters then try to lower the baudrate (ULP_RISCV_UART_BAUDRATE) using `menuconfig`.