Olimex's UEXT-CH390 Brings Wired Ethernet to Raspberry Pi RP2040, RP2350, and Espressif ESP32 Boards

Bulgarian open-hardware specialist Olimex has launched a Ethernet module designed to add wired Ethernet connectivity to Raspberry Pi RP2040, RP2350, and Espressif ESP32-based microcontroller boards — complete with examples for standalone and Home Assistant use via ESPHome, even on the Raspberry Pi RP2040.

"Wi-Fi is great, but sometimes people want to avoid it — to prevent wireless attacks to their home network, or to connect devices behind metal walls, basements etc. where Wi-Fi can't penetrate well," Olimex founder Tsvetan Usunov says of the thinking behind the new add-on board. "UEXT-CH390 is our new small open source hardware module that adds 100 Mbit Ethernet to any board with a UEXT or pUEXT connector. It uses the WCH [Electronics] CH390 SPI-to-Ethernet controller, measures only 50×21mm [around 1.97×0.83"], and has both the classic UEXT and the small 1.0mm pUEXT connectors."

The compact Fast Ethernet board is primarily designed with Olimex's own open-hardware development boards in mind, offering solder-free connectivity via either the classic full-size UEXT or compact pUEXT expansion connectors. When connected to a board with an Espressif ESP32-family microcontroller, the Ethernet port is fully supported in ESPHome — meaning it can be used to drive smart home devices connected to Home Assistant with ease.

"ESPHome supported CH390 only on [an Espressif] ESP32, not on [a Raspberry Pi] RP2040," Usunov notes. "So we wrote it. The example project on GitHub contains a version-pinned ESPHome Ethernet component with an RP2040 CH390 driver, built and tested with ESPHome 2026.9.0. Use it as a starting point for your own wired sensors, relays and controllers. Anything ESPHome can do on RP2040 now works over a cable. There is also a plain [Raspberry Pi] Pico SDK [Software Development Kit] example with lwIP [lightweight Internet Protocol] and a small web server if you prefer to work without ESPHome."

Getting the CH390 up and running over UEXT wasn't entirely straightforward, Usunov admits. "The CH390 interrupt and reset are not on UEXT, so the driver uses a software reset and polling," he explains of the workarounds required. "Overlapping SPI accesses from two tasks caused lockups. All link register reads now run inside the protected Ethernet task. ESPHome's API watchdog rebooted the board when Home Assistant was not connected. The watchdog is disabled for standalone use. BOOT1 shares the QSPI flash chip select line. It is read safely with the RAM-resident BOOTSEL helper."

The board is now available to pre-order from the Olimex store, priced at €4.95 (around $5.55) before volume discounts; as always, the hardware and firmware are both available on GitHub under the strongly reciprocal variant of the CERN Open Hardware License Version 2 and the permissive MIT license respectively.