Regarding MII spec, I have a question about the RX_DV signal that I'm not able to find elsewhere: From the perspective of the Ethernet MAC, when sending a frame to a PHY over MII, does the RX_DV (Data Valid) signal have to stay HIGH/asserted for the entire frame, or can it go LOW and back HIGH again on the same frame (and still be interpreted as 1 frame by the PHY)?

I guess another way of asking is: does an MII PHY use the RX_DV as a delimiter for individual frames, or does it check the SOF byte of something else to distinguish frames?

Any help is appreciated

1 Answer 1


The MII is specified in IEEE 802.3 Clause 22. You can get it for free after registration on IEEE GET.

RX_DV is asserted when the preamble is detected, and it is deasserted at the end of the frame, see Clause for details:

RX_DV (Receive Data Valid) is driven by the PHY to indicate that the PHY is presenting recovered and decoded nibbles on the RXD<3:0> bundle and that the data on RXD<3:0> is synchronous to RX_CLK. RX_DV shall transition synchronously with respect to the RX_CLK. RX_DV shall remain asserted continuously from the first recovered nibble of the frame through the final recovered nibble and shall be negated prior to the first RX_CLK that follows the final nibble. In order for a received frame to be correctly interpreted by the Reconciliation sublayer and the MAC sublayer, RX_DV must encompass the frame, starting no later than the Start Frame Delimiter (SFD) and excluding any End-of-Frame delimiter.

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