esp32-eth-serial

ESP32 ethernet to serial port bridge

Motivation

There are no readily available Ethernet to UART bridge having high bit rate with hardware flow control and low price. For example the EBYTE NT1 Ethernet bridge has maximum baud rate of 230400 only. The ESP32 on the other hand provides an excellent platform for Ethernet to UART bridge implementation. We use WT32-ETH01 'wireless tag' module. Yet the code may be easily ported to any other module having Ethernet interface.

Build environment

See https://docs.espressif.com/projects/esp-idf/en/latest/get-started/index.html

Building

git clone git@github.com:olegv142/esp32-eth-serial.git
cd esp32-eth-serial/src
idf.py set-target esp32
idf.py menuconfig
idf.py build

Configuring

The bridge has connection indicator output, serial data RX/TX lines and flow control lines RTS/CTS, the last one is optional and is not enabled by default. All pin locations can be configured by running idf.py menuconfig. Besides one can configure UART baud rate, buffer size and whether to use CTS flow control line. The supported serial baud rates are in the range from 9600 to 1843200 with 921600 being the default.

Flashing

Unless you have dev kit with USB programmer included you will need some minimal wiring made to the ESP32 module to be able to flash it. The following figure shows an example of such setup with programming connections shown in blue. The connections providing serial interface to your system are shown in black.

You can use virtually any USB-serial bridge capable of working at 115200 baud. The IO0 pin should be connected to the ground while powering up the module in order to turn it onto serial programming mode. After that you can issue idf.py flash command in the project directory and wait for the flashing completion. Then turn off power, disconnect programming circuit and enjoy using your brand new Ethernet to serial bridge.

Connections

You can use hardware flow control CTS/RTS lines or ignore them depending on your system design details. Basically not using RTS line is safe if packets you are sending to the module's RXD line are not exceeding 128 bytes. The CTS line usage is completely up to your implementation of the serial data receiver. If you are not going to use CTS line you should either connect it to the ground or disable at firmware build stage by means of idf.py menuconfig. The EN line plays the role of the reset to the module. Low level on this line turns the module onto the reset state with low power consumption. In case you are not going to use this line it should be pulled up. The pull up resistors on the TXD and RTS lines are needed to prevent them from floating during module boot.

Detailed description

The bridge does not have static network configuration. Its expecting to get network configuration via DHCP from network its connected to. There are two server sockets the bridge is listening on. The first one is 'echo socket' (3333 by default). Its used for testing exclusively. It just sends all data received from network back to the sender. The second one is 'bridge socket' (3142 by default). It sends all data received from network to UART and sends all data received from UART to network (to the other side of network connection). Once the connection is established to any of those sockets no other connection can be made to the same socket until the first one disconnects. Yet both sockets can serve connections simultaneously. The connection indicator output has high level while connection to bridge socket is established.

Testing

The esp32-eth-serial/test folder has scripts for testing both server sockets in echo mode. The echo_perf.sh script sends continuous stream of random data to echo socket and receives data back. The echo_test.sh sends chunks of random data to echo socket, receives them back and verify that data received is the same as data sent. The maximum throughput of the echo socket according to those tests is around 1.3 MBytes/sec.

The uart_echo_perf.sh script sends continuous stream of random data to bridge socket and receives data back. To run UART echo tests one should enable CTS flow control and connect RX to TX and RTS to CTS pins. Similarly the uart_echo_test.sh script sends chunks of random data to bridge socket, receives them back and verify that data received is the same as data sent.

Troubleshooting

The ESP32 module is using the same serial channel used for programming to print error and debug messages. So if anything goes wrong you can attach the programming circuit without grounding the IO0 pin and monitor debug messages by calling idf.py -p monitor.

Power consumption

130mA in idle state, up to 150mA while transferring data at maximum rate.

Framework version

The code was built with esp-idf version 5.2.1.

Notes on porting and code modifications

• The WT32-ETH01 used in this project has on-board clock generator with clock enable pin. Other platforms may not have such pin. One can run idf.py menuconfig to choose if clock enable pin is used.
• More server sockets may be easily added to main/tcp_server.c. One can add yet another bridge socket with second UART if necessary.