6.4.1 Media Access Controller

The Transmit Block of the Media Access Controller (MAC) takes data from FIFO, adds preamble, checks and adds padding and frame check sequence (FCS). Only 10Mbps half duplex Ethernet mode of operation is supported.

Operating in half duplex mode, the MAC Transmit Block generates data according to the Carrier Sense Multiple Access with Collision Detect (CSMA/CD) protocol. The start of transmission is deferred if Carrier Sense (CRS) is active. If Collision (COL) is detected during transmission, a jam sequence is asserted and the transmission is retried after a random back off.

The Receive Block of the MAC checks for valid preamble, Frame Check Sequence (FCS), alignment/length, and presents received frames to the MAC address checking block and FIFO. It can optionally strip FCS bytes from the received frame before transferring it to FIFO.

The Address Checker recognizes four specific 48-bit addresses, can recognize four different types of ID values, and contains a 64-bit Hash register for matching multicast and unicast addresses as required. It can recognize the broadcast address all-'1' (0xFFFFFFFFFFFF) and copy all frames (promiscuous mode). The MAC can also reject all frames that are not VLAN tagged.