PM3351-SW datasheet(92/268 Pages) PMC

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PM3351 ELAN 1X100

DATA SHEET

PMC-970113

ISSUE 3

SINGLE PORT FAST ETHERNET SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

The 16-bit MulticastMask field is used to restrict broadcasts, multicasts and floods of

packets received on the MAC channel with which this local port descriptor is

associated. The 8 LSBs of this mask are always set to zero on the ELAN 1x100, while

the upper 8 bits of the mask correspond on a one-to-one basis to the eight possible

devices in the system. A device is permitted to be part of a multicast if its corresponding

bit in the MulticastMask field is set to a '1'. (Note that the bit in the MulticastMask

corresponding to the ELAN 1x100 device itself is always set to zero at initialization

time.) The MulticastMask field is logically ANDed with the global restriction mask

managed by the spanning tree protocol entity to permit broadcasts to be further

restricted to remove loops in the broadcast topology.

A 16-bit BackoffMask field is provided to allow the range of collision backoff values to

be adjusted via management access. The BackoffMask field contents are logically

ANDed with the standard pseudorandom backoff value generated according to the

IEEE 802.3 backoff timer computation algorithm. The uppermost 6 bits of the

BackoffMask must be set to zero. The lower 10 bits should normally be set to all-ones,

if the IEEE 802.3 standard backoff algorithm (which specifies a range of 0 to 1023 slot

times, inclusive) is to be adhered to; however, smaller ranges of backoff values may be

generated by reducing the number of '1' bits set in this mask.

The PortFlags field supplies several port-specific control bits that are used to determine

the operating mode of the switching firmware when handling Ethernet frames that are

received from or transmitted to this port. This field is formatted as:

43210

BackPEn

Special

Type

reserved

TXBlkd

RXBlkd

BackPRn

BackPEn:

If set, indicates that backpressure flow control is enabled for this port. This bit

is normally set via a configuration parameter at system initialization time.

Special:

If set, indicates that the data descriptors created for Ethernet frames received

on this MAC channel should have the Special flag bits set in their Flags fields,

causing them to be specially handled by the switching firmware. Normally set

to zero.

Type:

This 2-bit field is used to initialize the 2-bit frame Type subfield in the Flags

fields of data descriptors corresponding to Ethernet frames received on this

MAC channel. Normally set to 01 binary, corresponding to unicast frames

received on a local MAC channel.

TXBlkd:

If set, indicates that the MAC channel is blocked to transmit; i.e., no frames

PM3351-SW Datasheet(PDF) 92 Page - PMC-Sierra, Inc

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