line, the PHY has to provide the MDIO signal between 0 and 300 ns after the rising edge of the clock. Hence, with a minimum clock period of 400 ns (2.5 MHz maximum clock rate) the MAC can safely sample MDIO during the second half of the low cycle of the clock. MDIO Packet Format (clause 22) PRE_32 The first field in the MDIO header is the Preamble. During the preamble, the MAC sends 32 bits, all '1', on the MDIO line. ST The Start field consists of 2 bits and always contains the combination '01'. OP The Opcode consists of 2 bits. There are two possible opcodes, read '10' or write '01'. PA5 5 bits, PHY address. RA5 The Register Address field indicates the register to be written to or read from. It is 5 bits long. TA The turn-around field is 2 bits long. When data is being written to the PHY, the MAC writes '10' to the MDIO line. When data is being read, the MAC releases the MDIO line. D16 16 bits, data. This can be sent by either the SME or the PHY, depending on the value of the OP field. Z Tristate MDIO. Commands IEEE 802.3 Part 3 use different opcodes and start sequences. Opcodes 00(set address) and 11(read)/01(write)/10(read increment) are used as two serial transactions to read and write registers. References External links Clause 22 Access to Clause 45 Registers Ethernet standards