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156 Software Developer’s Manual
Ethernet Interface
8.2.1 Internal SerDes Interface/TBI Mode– 1Gb/s
1
The 82546GB/EB and 82545GM/EM Ethernet controllers contain one or two internal SerDes
devices (depending whether or not they support one or two ports). The MAC communicates with
the SerDes over a TBI interface. Normally, this interface is not exposed externally.
For the
82554GC/EI, TBI mode is selectable via an external pin TBI-MODE. Software cannot
override this pin. This interface has 125 Mb/s 10-bit data paths for both receive and transmit. The
clock at the transmit interface operates at 125 MHz; the receive interface has two clocks running at
62.5 MHz that are 180 degrees out of phase as follows:
• RX_DATA: 10-bit receive data bus
• TX_DATA: 10-bit transmit data bus
• RBC0/I_RBC1: Receive clocks (62.5 MHz; 180 degree phase shift between I_RBC0 and
I_RBCI)
• GTX_CLK: Transmit clock (125 MHz)
8.2.1.1 Gigabit Physical Coding Sub-Layer (PCS) for the Internal SerDes
2
The Ethernet controller integrates the 802.3z PCS function on-chip. The on-chip PCS circuitry is
used when the link interface is configured for internal SerDes mode and is bypassed in internal
PHY mode.
The packet encapsulation is based on the Fibre Channel physical layer (FC0/FC1) and uses the
same coding scheme to maintain transition density and DC balance. The physical layer device is a
SerDes and is used for 1000BASE-SX, -LX, or -CX configurations.
8.2.1.2 8B10B Encoding/Decoding
The Gigabit PCS circuitry uses the same transmission coding scheme used in the Fibre Channel
physical layer specification. The 8B10B coding scheme was chosen by the IEEE standards
committee in order to provide a balanced, continuous stream with sufficient transition density to
allow for clock recovery at the receiving station. There is a 25 percent overhead for this
transmission code which accounts for the data signaling rate of 1250 Mb/s with 1000 Mb/s of
actual data.
1. TBI (10-Bit Interface) - 1GB/s for the 82554GC/EI.
2. Gigabit Physical Coding Sub-Layer (PCS) for TBI (82554GC/EI).