40.6.1 Media Access Controller

The Media Access Controller (MAC) transmit block takes data from FIFO, adds preamble and, if necessary, pad and frame check sequence (FCS). Both Half Duplex and Full Duplex Ethernet modes of operation are supported. When 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) becomes active during transmission, a jam sequence is asserted and the transmission is retried after a random backoff. The CRS and COL signals have no effect in Full Duplex mode.

The MAC receive block checks for valid preamble, FCS, alignment and length, and presents received frames to the MAC address checking block and FIFO. Software can configure the GMAC to receive jumbo frames up to 10240 bytes. It can optionally strip CRC from the received frame prior to transfer to FIFO.

The address checker recognizes four specific 48-bit addresses, can recognize four different type ID values, and contains a 64-bit Hash register for matching multicast and unicast addresses as required. It can recognize the broadcast address of all ones and copy all frames. The MAC can also reject all frames that are not VLAN tagged and recognize Wake on LAN events.

The MAC receive block supports offloading of IP, TCP and UDP checksum calculations (both IPv4 and IPv6 packet types supported), and can automatically discard bad checksum frames.

The MAC replaces the timestamp field in PTP 1588 transmit sync frames to support One-Step Clock mode.

The MAC does one-step transparent clock residence time correction for PTP 1588 version 2 transmit sync frames.