Freescale Semiconductor MPC8313E PowerQUICC II Pro Family Reference Manual page 840

Enhanced Three-Speed Ethernet Controllers
15.5.4 Ten-Bit Interface (TBI)
This section describes the ten-bit interface (TBI) and the TBI MII set of registers.
15.5.4.1 TBI Transmit Process
The eTSEC's TBI implements the transmit portion of the physical coding sublayer as found in Clause 36
of IEEE 802.3z. In SerDes mode, packets conveyed across the GMII are encapsulated and encoded into
10-bit symbols and output to the SerDes. In GMII mode, the GMII signals are passed through to the
attached GMII PHY.
15.5.4.1.1 Packet Encapsulation
If TX_EN is de-asserted the eTSEC outputs an idle stream. If TX_EN is asserted, a Start_of_Packet
symbol is output. This symbol replaces the first byte of the preamble field. All other bytes of the packet
pass through an 8B10B encoding module. After the last byte of the FCS field is signaled through the GMII,
the MAC de-asserts TX_EN. The eTSEC then outputs an End_of_Packet symbol. Then, depending on the
position of the End_of_Packet symbols (being in either an odd or even position) the eTSEC outputs one
or two Carrier_Extend symbols. Following the last Carrier_Extend symbol, the eTSEC resumes sending
idle codes. If, during a packet, the eTSEC wishes to mark a byte invalid, TX_ER is asserted. The eTSEC,
upon detection of TX_ER, substitutes the data symbol for an Error_Propagation symbol.
15.5.4.1.2 8B10B Encoding
Every eight-bit data octet has two (not necessarily different) ten-bit symbols associated with it. Depending
on the running disparity (the cumulative difference of ones and zeroes) the eTSEC module chooses the
appropriate symbol.
Special encapsulation symbols are called ordered_sets. Ordered_sets are comprised of one to four ten-bit
symbols. Ordered_sets can be found in Clause 36 of the IEEE 802.3z specification.
15.5.4.1.3 Preamble Shortening
Because the idle ordered_set comprises two symbols and begins on an even symbols boundary, packets
can only begin on an even boundary. However, the GMII has no such restriction and may signal TX_EN
on an odd boundary. If this happens, the eTSEC delays the Start_of_Packet symbol, effectively ignoring
the first byte of preamble; thus, a seven octet preamble becomes six octets on the Ten-Bit Interface.
15.5.4.2 TBI Receive Process
The eTSEC's TBI Implements the receive portion of the physical coding sublayer as found in Clause 36
of IEEE 802.3z. The Receive portion includes the Synchronization state machine. In SerDes mode, the
eTSEC first attempts to acquire synchronization on the link by examining received symbols. Once
synchronization is acquired, received packets are decoded and sent across the Receive GMII interface. In
GMII mode, the GMII signals are passed through to the MAC.
MPC8313E PowerQUICC™ II Pro Integrated Processor Family Reference Manual, Rev. 2
15-122 Freescale Semiconductor