signal from the CRS_DV line in half-duplex mode. This means a slight modification of the definition of CRS: On MII, CRS is asserted for both Rx and Tx frames; on RMII, only for Rx frames. This has the consequence that on RMII, the two error conditions no carrier and lost carrier cannot be detected, and it is difficult or impossible to support shared media such as 10BASE2 or 10BASE5.

Since the RMII standard neglected to stipulate that TX_EN should only be sampled on alternate