Ethernet & PoE on Embedded Systems Development

Ethernet PHY, MAC & RMII/MII

Mon, 01 Jan 0001 00:00:00 +0000

Ethernet PHY, MAC & RMII/MII#

Wired Ethernet on a microcontroller requires two functional blocks: a MAC (Media Access Controller) that handles frame construction, CRC generation, and DMA transfers, and a PHY (Physical Layer Transceiver) that converts digital signals to analog levels on the wire. Some MCUs integrate the MAC as a peripheral — STM32F4/F7/H7, i.MX RT1060, ESP32, SAM E70 — while others lack Ethernet entirely and rely on SPI-attached modules (covered separately). The PHY is almost always a discrete external chip, connected to the MAC through either the MII or RMII interface.

SPI Ethernet Modules

Mon, 01 Jan 0001 00:00:00 +0000

SPI Ethernet Modules#

Not every MCU has an integrated Ethernet MAC. Many popular platforms — RP2040, STM32F1/L4, nRF52, ATmega — lack native Ethernet. SPI-attached Ethernet controllers bridge this gap by providing a complete MAC + PHY + magnetics interface accessible over a standard 4-wire SPI bus. Two chips dominate this space: the WIZnet W5500, which offloads the entire TCP/IP stack into hardware, and the Microchip ENC28J60, which provides only raw Ethernet frame access and relies on the host MCU to run a software TCP/IP stack like lwIP.

lwIP & Embedded TCP/IP Stacks

Mon, 01 Jan 0001 00:00:00 +0000

lwIP & Embedded TCP/IP Stacks#

lwIP (lightweight IP) is the dominant TCP/IP stack for bare-metal and RTOS-based microcontrollers. Written by Adam Dunkels and maintained as an open-source project, it runs on platforms with as little as 40 KB of RAM and provides a functional IPv4/IPv6 stack with TCP, UDP, DHCP, DNS, ICMP, IGMP, and ARP. ESP-IDF, STM32 CubeMX, NXP MCUXpresso, and TI SimpleLink all ship lwIP as their default network stack. Understanding its internal architecture — particularly the three API levels, memory management, and RTOS integration — is essential for debugging the throughput bottlenecks, memory leaks, and connection failures that inevitably arise in embedded networking.

Power over Ethernet: PD Design

Mon, 01 Jan 0001 00:00:00 +0000

Power over Ethernet: PD Design#

Power over Ethernet (PoE) delivers both data and DC power over standard Ethernet cabling, eliminating the need for a separate power supply at the device. For embedded systems — IP cameras, access points, industrial sensors, building automation controllers — PoE simplifies installation to a single cable drop. The Powered Device (PD) is the equipment that receives power from the cable. Designing the PD side involves selecting the correct IEEE 802.3 power class, choosing a PD controller IC, implementing the detection and classification handshake, converting 48 V to the board’s operating voltages, meeting isolation requirements, and managing thermal dissipation.

Link Management & Diagnostics

Mon, 01 Jan 0001 00:00:00 +0000

Link Management & Diagnostics#

An Ethernet link is not a fire-and-forget connection. Cables get unplugged, switches reboot, duplex mismatches degrade throughput silently, and long cable runs develop faults over time. On MCU-based devices that often run unattended for months or years, robust link management — detecting link state changes, diagnosing failures, and recovering automatically — is as important as getting the initial connection working. The PHY provides the raw link status and diagnostic capabilities through MDIO registers; the firmware must poll or react to these signals and take appropriate action.