STM32 microcontrollers use an external PHY connected via RMII or MII interfaces to enable Ethernet connectivity, requiring proper pin configuration and PHY addressing for network communication.

Hardware Interface

STM32 Ethernet relies on a MAC (Media Access Controller) integrated into the microcontroller and an external PHY (Physical Layer) chip such as LAN8742, DP83848, or LAN8720, which handles electrical signaling, link detection, and speed negotiation . The PHY connects to the STM32 via either:

• RMII (Reduced Media Independent Interface): Uses 9 pins, simpler and common on most STM32 boards, including Nucleo and Discovery series .
• MII (Media Independent Interface): Uses 18 pins, more complex, less commonly used due to higher pin count . The PHY address must be correctly set (typically 0 for onboard PHYs) to ensure proper communication between the MAC and PHY . Pull-down resistors on PHY pins like PHYAD0 and nINTSEL are used to configure the address and reference clock output.

Pin Configuration

For RMII, the nine essential pins include:

• TXD0, TXD1 (Transmit data)
• RXD0, RXD1 (Receive data)
• TX_EN (Transmit enable)
• CRS_DV (Carrier sense / data valid)
• REF_CLK (Reference clock)
• MDIO (Management data input/output)
• MDC (Management data clock) All pins must be configured in alternate function mode in STM32CubeMX or manually in firmware to enable Ethernet functionality .

Software Setup

1. CubeMX Configuration: Enable the Ethernet peripheral, select RMII or MII mode, and assign the correct pins and PHY address .
2. LWIP Stack: Use the Lightweight IP (LWIP) stack to implement TCP/IP communication, allowing STM32 to act as a client, server, or host HTTP/MQTT services .
3. IP Configuration: Ethernet interfaces can be configured with DHCP or static IP using systemd-networkd or ifconfig on STM32MPU platforms .

Practical Considerations

• Ensure the PHY type and interface match the board schematic; mismatches prevent link establishment .
• RMII is preferred for simplicity and fewer pins, while MII may be used for legacy or high-speed designs .
• Ethernet is ideal for high-throughput, long-distance communication, and reliable LAN connectivity compared to UART, SPI, or I2C . By correctly wiring the PHY, configuring pins, and setting up the software stack, STM32 devices can achieve robust Ethernet connectivity suitable for IoT, industrial, and embedded network applications .