DP83820 datasheet(18/87 Pages) NSC

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3.0 Functional Description (Continued)

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3.13 Buffer Management

The buffer management scheme used on the DP83820

allows quick, simple and efficient use of the frame buffer

memory. Frames are saved in similar formats for both

transmit and receive. The buffer management scheme also

uses

separate

buffers

and

descriptors

for

packet

information. This allows effective transfers of data from the

receive buffer to the transmit buffer by simply transferring

the descriptor from the receive queue to the transmit

queue.

The format of the descriptors allows the packets to be

saved in a number of configurations. A packet can be

stored in memory with a single descriptor and a single

packet fragment, or multiple descriptors with single

fragments. This flexibility allows the user to configure the

DP83820 to maximize efficiency. Architecture of the

specific system’s buffer memory, as well as the nature of

network

traffic,

will

determine

the

most

suitable

configuration of packet descriptors and fragments.

3.13.1 Overview

The buffer management design has the following goals:

— simplicity

— efficient use of the PCI bus (the overhead of the buffer

management technique is minimal),

— low CPU utilization,

— flexibility.

Descriptors may be either per-packet or per-packet-

fragment. Each descriptor may describe one packet

fragment.

Receive

and

transmit

descriptors

are

symmetrical.

3.13.2 Descriptor Format

DP83820 uses a symmetrical format for transmit and

receive descriptors. In bridging and switching applications

this symmetry allows software to forward packets by simply

moving the list of descriptors that describe a single

received packet from the receive list of one MAC to the

transmit list of another. Descriptors must be aligned on a

64-bit boundary.

TXPktsErrored

RFC 1213

software, increment on

receive packets with

cmdsts.TXA set

This counter is incremented for each packet

encountering errors during transmission.

This count does include transmissions

aborted manually and due to FIFO

underruns, but does not include packets

which experience less than 16 in-window

collisions.

TXExcessiveCollisions

RFC 1643,

802.3 LM

software, increment on

transmit packets with

cmdsts.EC set.

This counter is incremented for each

transmission aborted after experiencing 16

in-window collisions.

TXExcessiveDeferral

802.3 LM

software, increment on

transmit packets with

cmdsts.ED set.

This counter is incremented for each

transmission aborted due to a time-out of

the excessive deferral timer (3.2ms).

TXOWC

RFC 1643,

802.3 LM

software, increment on

transmit packets with

cmdsts.OWC set.

This counter is incremented for each

transmission which is aborted due to an

out-of-window collision.

TXCSErrors

RFC 1643,

802.3 LM

software, increment on

transmit packets with

cmdsts.CRS set.

This counter is incremented for each

transmission on which carrier is not

detected after the start of transmission, or

carrier sense is lost during transmission.

TXSQEErrors

RFC 1643

hardware, see

MIB:TxSQEErrors

This counter is incremented when the

collision heartbeat pulse is not detected

from by the PMD after a transmission.

DP83820 Datasheet(PDF) 18 Page - National Semiconductor (TI)

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