Motorola MPC8260 - Manual (Page 883)

MOTOROLA Chapter 30. Fast Ethernet Controller 30-9
Part IV. Communications Processor Module
30.8 Ethernet Parameter RAM
For Ethernet mode, the protocol-speciÞc area of the FCC parameter RAM is mapped as in
Table 30-2.
Table 30-2. Ethernet-Specific Parameter RAM
Offset 1 Name Width Description
0x3C STAT_BUF Word Buffer of internal usage
0x40 CAM_PTR Word CAM address
0x44 C_MASK Word Constant MASK for CRC (initialize to 0xDEBB_20E3). For the 32-bit CRC-CCITT.
0x48 C_PRES Word Preset CRC (initialize to 0xFFFF_FFFF). For the 32-bit CRC-CCITT.
0x4C CRCEC 2 Word CRC error counter. Counts each received frame with a CRC error. Does not count
frames not addressed to the station, frames received in the out-of-buffers condition,
frames with overrun errors, or frames with alignment errors.
0x50 ALEC 2 Word Alignment error counter. Counts frames received with dribbling bits. Does not count
frames not addressed to the station, frames received in the out-of-buffers condition,
or frames with overrun errors.
0x54 DISFC 2 Word Discard frame counter. Incremented for discarded frames because of an out-of-
buffers condition or overrun error. The CRC need not be correct for this counter to be
incremented.
0x58 RET_LIM Hword Retry limit (typically 15 decimal). Number of retries that should be made to send a
frame. If the frame is not sent after this limit is reached, an interrupt can be
generated.
0x5A RET_CNT Hword Retry limit counter. Temporary decrementer used to count retries made.
0x5C P_PER Hword Persistence. Allows the Ethernet controller to be less persistent after a collision.
Normally cleared, P_PER can be from 0 to 9 (9 = least persistent). The value is
added to the retry count in the backoff algorithm to reduce the chance of
transmission on the next time-slot. Using a less persistent backoff algorithm
increases throughput in a congested Ethernet LAN by reducing the chance of
collisions. FPSMR[SBT] can also reduce persistence of the Ethernet controller. The
Ethernet/802.3 speciÞcations permit the use of P_PER.
0x5E BOFF_CNT Hword Backoff counter
0x60 GADDR_H Word Group address Þlters high and low are used in the hash table function of the group
addressing mode. The user may write zeros to these values after reset and before
the Ethernet channel is enabled to disable all group hash address recognition
functions. The
SET GROUP ADDRESS command is used to enable the hash table. See
Section 30.13, ÒHash Table Algorithm.Ó
0x64 GADDR_L Word
0x68 TFCSTAT Hword Out-of-sequence TxBD. Includes the status/control, data length, and buffer pointer
Þelds in the same format as a regular TxBD. Useful for sending ßow control frames.
This areaÕs TxBD[R] is always checked between frames, regardless of
FPSMRx[FCE]. If it is not ready, a regular frame is sent. The user must set TxBD[L]
when preparing this BD. If TxBD[I] is set, a TXC event is generated after frame
transmission. This area should be cleared when not in use.
0x6A TFCLEN Hword
0x6C TFCPTR Word
0x70 MFLR Hword Maximum frame length register (typically1518 decimal). If the Ethernet controller
detects an incoming frame exceeding MFLR, it sets RxBD[LG] (frame too long) in the
last RxBD, but does not discard the rest of the frame. The controller also reports the
frame status and length of the received frame in the last RxBD. MFLR includes all in-
frame bytes between the start frame delimiter and the end of the frame.