4.3 Serial Management Interface (SMI)

The Serial Management Interface (SMI) is used to control the device and obtain its status. This interface supports the standard PHY registers required by Clause 22 of IEEE Std 802.3, as well as “vendor-specific” registers allowed by the specification. Unimplemented registers will be read as hexadecimal “0000”. Device registers are detailed in the Register Descriptions section.

At the system level, SMI provides two signals: MDIO and MDC. The MDC signal is an aperiodic clock provided by the station management entity (STA) host controller. MDIO is a bi-directional data SMI input/output signal that receives serial data (commands) from the STA and sends serial data (status) to the STA. The minimum time between edges of the MDC is 100 ns. There is no maximum time between edges. The minimum cycle time (i.e., the time between two consecutive rising or two consecutive falling edges) is 250 ns. These modest timing requirements allow this interface to be easily driven by the I/O port of a microcontroller. MDIO should be connected to VDDP via a pull-up resistor.

The data on the MDIO line is latched on the rising edge of the MDC. The management frame structure and timing is shown in the following figures. The timing relationships of the MDIO signals are further described in the SMI Timing section of the Operational Characteristics AC Specifications.