Chapter 19. Ethernet Media Access Controller
The PPC405GP Ethernet media access controller (EMAC) is a generic implementation of the
Ethernet Media Access
Control (MAC) protocol complying with ANSI/IEEE Std 802.3 and IEEE
802.3u supplement.
EMAC supports half-duplex (CSMAlCD) and full-duplex operation for both
10-Mbps and 100-Mbps operations.
EMAC provides two on-chip
peripheral bus (OPB) slave interfaces. The first OPB interface provides
access to EMAC configuration and status registers. The
PLB/OPB bridge allows the 405 processor
core to access these registers.
The second
OPB interface is used to exchange packet information with the memory access
layer (MAL). The MAL is a multi-channel, intermediate hardware layer that resides between packet-
based communication macros (such as EMAC) and
external memory (such as SDRAM or SRAM).
The MAL transfers packet information and packet status between the EMAC and
external memory
separately for each of the three EMAC channels (one receive and two transmit). Software (a device
driver) is
responsible for maintaining a buffer descriptor ring and a set of data buffers
in
external
memory for each of the channels, and manages the exchange of packet data between the data
buffers and the software
protocol.
The MAL performs functions such as arbitration between service requests, handling the buffer
descriptor memory structure, updating the descriptor status/control
fields at the end of packet
transfer, and so on. EMAC supports
unlimited burst length transactions on the MAL interface.
EMAC utilizes its media independent interface
(Mil) to communicate with standard physical interface
devices (pHYs).
EMAC
utilizes independent Receive (RX) and Transmit (TX) FIFOs. Programmable FIFO thresholds
minimize overflows and underruns and can launch integrated, IEEE 802.3x Pause Packets for flow
control.
As part of the Remote Monitoring (RMON) and Management Information Base (MIB) defined in IEEE
802.3z
the EMAC contains registers that count the number of octets transmitted and received.
Preliminary Ethernet Media Access Controller
19-1
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