The MDIO interface for 1 Gbps operation (and slower speeds) is defined in IEEE
802.3-2008, clause 22. The following figure shows an example MDIO bus system. This
two-wire interface consists of a clock (MDC) and a shared serial data line (MDIO). The
maximum permitted frequency of Management Data Clock (MDC) is set at 2.5 MHz. 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 of which are MDIO slaves (MDIO
Managed Device (MMD) entities).
Figure 1. A Typical MDIO-Managed System
The MDIO bus system is a standardized interface for accessing the
configuration and status registers of Ethernet PHY devices. In the example shown, the
Management Host Bus I/F of the Ethernet MAC is able to access the configuration and
status registers of two PHY devices using the MDIO bus.