The Serial Management Interface (SMI) provides access to the DP83TC815-Q1 internal register space for status information and configuration. The SMI frames and base registers are compatible with IEEE 802.3 clause 22. The implemented register set consists of the registers required by the IEEE 802.3 plus several others to provide additional visibility and controllability of the DP83TC815-Q1. Additionally, the DP83TC815-Q1 includes control and status registers added to clause 45 as defined by IEEE 802.3bw. Access to clause 45 register field is achieved using clause 22 access.

The SMI includes the management clock (MDC) and the management input and output data pin (MDIO). MDC is sourced by the external management entity, also called Station (STA), and can run at a maximum clock rate of 24MHz. MDC is not expected to be continuous, and can be turned off by the external management entity when the bus is idle.

MDIO is sourced by the external management entity and by the PHY. The data on the MDIO pin is latched on the rising edge of the MDC. MDIO pin requires a pullup resistor (2.2K?), which pulls MDIO high during IDLE and turnaround.

Up to 9 DP83TC815-Q1 PHYs can share a common SMI bus. To distinguish between the PHYs, a 4-bit address is used. During power-up-reset, the DP83TC815-Q1 latches the PHYAD[3:0] configuration pins to determine its address.

The management entity must not start an SMI transaction in the first cycle after power-up-reset. To maintain valid operation, the SMI bus must remain inactive at least one MDC cycle after hard reset is deasserted. In normal MDIO transactions, the register address is taken directly from the management-frame reg_addr field, thus allowing direct access to 32 16-bit registers (including those defined in IEEE 802.3 and vendor specific). The data field is used for both reading and writing. The Start code is indicated by a <01> pattern. This pattern makes sure that the MDIO line transitions from the default idle line state. Turnaround is defined as an idle bit time inserted between the Register Address field and the Data field. To avoid contention during a read transaction, no device can actively drive the MDIO signal during the first bit of turnaround. The addressed DP83TC815-Q1 drives the MDIO with a zero for the second bit of turnaround and follows this with the required data.

For write transactions, the station-management entity writes data to the addressed DP83TC815-Q1, thus eliminating the requirement for MDIO Turnaround. The turnaround time is filled by the management entity by inserting <10>.