The DP83826 SMI function supports read and write access to the extended register set using the Register Control Register (REGCR, address 0x000D), the Data Register (ADDAR, address 0x000E), and the MDIO Manageable Device (MMD) indirect method defined in IEEE 802.3ah draft for Clause 22 for accessing the extended register set.

The standard register set, MDIO registers 0 to 31, is accessed using the normal direct-MDIO access or the indirect method, except for register REGCR and register ADDAR, which are accessed only using the normal MDIO transaction. The SMI function ignores indirect access to these registers.

REGCR is the MMD access control. In general, register REGCR[4:0] is the device address DEVAD that directs any accesses of the ADDAR register to the appropriate MMD.

The DP83826 supports three MMD device addresses:

1. The Vendor-Specific device address DEVAD[4:0] = 11111 is used for general MMD register accesses.
2. DEVAD[4:0] = 00011 is used for Energy Efficient Ethernet MMD register accesses. Register names for registers accessible at this device address are preceded by MMD3.
3. DEVAD[4:0] = 00111 is used for Energy Efficient Ethernet MMD registers accesses. Register names for registers accessible at this device address are preceded by MMD7.

All accesses through register REGCR and ADDAR must use the correct DEVAD. Transactions with other DEVAD are ignored. REGCR[15:14] holds the access function: address (00), data with no post increment (01).

• ADDAR is the address/data MMD register. ADDAR is used in conjunction with REGCR to provide the access to the extended register set. If register REGCR[15:14] is (00), then ADDAR holds the address of the extended address space register. Otherwise, ADDAR holds the data as indicated by the contents of its address register. When REGCR[15:14] is set to (00), accesses to register ADDAR modify the extended register set address register. This address register must always be initialized in order to access any of the registers within the extended register set.
• When REGCR[15:14] is set to (01), accesses to register ADDAR access the register within the extended register set selected by the value in the address register.

The following sections describe how to perform operations on the extended register set using register REGCR and ADDAR. The descriptions use the device address for general MMD register accesses (DEVAD[4:0] = 11111). For register accesses to the MMD3 or MMD7 registers the corresponding device address would be used.