The PHY Maintenance register is implemented as a shift register. Writing to the register starts a shift operation which is signalled as complete when bit two is set in the Network Status register (about 2000 MCK cycles later when bits 18:16 are set to 010 in the Network Configuration register). An interrupt is generated as this bit is set.
During this time, the MSB of the register is output on the MDIO pin and the LSB updated from the MDIO pin with each Management Data Clock (MDC) cycle. This causes the transmission of a PHY management frame on MDIO. See section 22.2.4.5 of the IEEE 802.3 standard.
Reading during the shift operation will return the current contents of the shift register. At the end of the management operation the bits will have shifted back to their original locations. For a read operation the data bits are updated with data read from the PHY. It is important to write the correct values to the register to ensure a valid PHY management frame is produced.
The Management Data Clock (MDC) should not toggle faster than 2.5 MHz (minimum period of 400 ns), as defined by the IEEE 802.3 standard. MDC is generated by dividing down MCK. Three bits in the Network Configuration register determine by how much MCK should be divided to produce MDC.