All transactions, read or write, are initiated by the MDIO master. All MDIO slave devices must respond when addressed. MDIO transactions take the form of an MDIO frame, containing fields for the transaction type, address and data. This MDIO frame is transferred across the MDIO wire synchronously to MDC. The abbreviations used in this section are explained in the following table.
Abbreviation | Term |
---|---|
PRE | Preamble |
ST | Start of frame |
OP | Operation code |
PHYAD | Physical address |
REGAD | Register address |
TA | Turnaround |
Write Transaction
The previous figure shows a Write transaction across the MDIO which is defined by OP = 01. The addressed PHY (PHYAD) device takes the 16-bit word in the data field and writes it to the register at REGAD.
Read Transaction
The previous figure shows a Read transaction, defined by OP = 10. The addressed PHY (PHYAD) device returns the 16-bit word from the register at REGAD. For details of the register map of PHY layer devices and a fuller description of the operation of the MDIO interface itself, see IEEE 802.3-2008 specification.