The Management Data Input/Output (MDIO) interface for access to Ethernet PHY devices for 1 Gbps operation and slower speeds is defined in IEEE 802.3, clause 22. This two-wire interface consists of a Management Data Clock (MDC) and a shared serial data line (MDIO). The maximum permitted frequency of MDC is set at 2.5 MHz. The following figure illustrates an example MDIO bus system.
An Ethernet MAC is shown as the MDIO bus master (the Station Management (STA) entity). Two PHY devices are shown connected to the same bus, both devices are MDIO slaves (MDIO Managed Device (MMD) entities).
The MDIO bus system is a standardized interface for accessing the configuration and status registers of Ethernet PHY devices. In the example illustrated, the Management Bus Interface of the Ethernet MAC is able to access the Configuration and Status registers of two PHY devices using the MDIO bus.