AM79C976 datasheet(67/309 Pages) AMD

67 / 309 page

8/01/00

Am79C976

P R E L I M INARY

The receiver section of the MAC engine will detect the

incoming preamble sequence when the RX_DV signal

is activated by the external PHY. The MAC will discard

the preamble and begin searching for the SFD except

in the case of 100BASE-T4, for which there is no pre-

amble. In that case, the SFD will be the first two nibbles

received. Once the SFD is detected, all subsequent

nibbles are treated as part of the frame. The MAC en-

gine will inspect the length field to ensure minimum

frame size, strip unnecessary pad characters (if auto-

matic pad stripping is enabled), and pass the remaining

bytes through the receive FIFO to the host. If pad strip-

ping is performed, the MAC engine will also strip the re-

ceived FCS bytes, although normal FCS computation

and checking will occur. Note that apart from pad strip-

ping, the frame will be passed unmodified to the host.

If the length field has a value of 46 or greater, all frame

bytes including FCS will be passed unmodified to the

receive buffer, regardless of the actual frame length.

If the frame terminates or suffers a collision before 64

bytes of information (after SFD) have been received,

the MAC engine will automatically delete the frame

from the receive FIFO, without host intervention. The

Am79C976 controller has the ability to accept runt

packets for diagnostic purposes and proprietary net-

works.

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The first 6 bytes of information after SFD will be inter-

preted as the destination address field. The MAC en-

gine provides facilities for physical (unicast), logical

(multicast), and broadcast address reception.

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The MAC engine provides several facilities which count

and recover from errors on the medium. In addition, it

protects the network from gross errors due to inability

of the host to keep pace with the MAC engine activity.

On completion of transmission, the MAC engine up-

dates various counters that are described in the Statis-

tics Counters section. The host CPU can read these

counters at any time for network management pur-

poses.

The MAC engine also attempts to prevent the creation

of any network error due to the inability of the host to

service the MAC engine. During transmission, if the

host fails to keep the transmit FIFO filled sufficiently,

causing an underflow, the MAC engine will guarantee

the message is sent with an invalid FCS, which will

cause the receiver to reject the message.

The MAC engine can be programmed to try to transmit

the same frame again after a FIFO underflow or exces-

sive collision error.

The status of each receive message is available in the

appropriate Receive Message Descriptor (RMD). All

received frames are passed to the host regardless of

any error.

During the reception, the FCS is generated on every

nibble (including the dribbling bits) coming from the MII,

although the internally saved FCS value is only up-

dated on each byte boundary. The MAC engine will ig-

nore an extra nibble at the end of a message, which

corresponds to dribbling bits on the network medium. A

framing or alignment error is reported to the user if an

FCS error is detected and there is an extra nibble in the

message. If there is an extra nibble but no FCS error,

no framing error is reported.

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The basic requirement for all stations on the network is

to provide fairness of channel allocation. The IEEE

802.3/Ethernet protocols define a media access mech-

anism which permits all stations to access the channel

with equality. Any node can attempt to contend for the

channel by waiting for a predetermined time (Inter

Packet Gap) after the last activity, before transmitting

on the media. The channel is a multidrop communica-

tions media (with various topological configurations

permitted), which allows a single station to transmit and

all other stations to receive. If two nodes simulta-

neously contend for the channel, their signals will inter-

act causing loss of data, defined as a collision. It is the

responsibility of the MAC to attempt to avoid and to re-

cover from collisions.

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The IEEE/ANSI 802.3 standard (ISO/IEC 8802-3 1990)

requires that the CSMA/CD MAC monitor the medium

for traffic by watching for carrier activity. When carrier is

detected, the media is considered busy, and the MAC

should defer to the existing message.

The ISO 8802-3 (IEEE/ANSI 802.3) standard allows an

optional two-part deferral after a receive message.

See ANSI/IEEE Std 802.3-1993 Edition, 4.2.3.2.1:

Note: It is possible for the PLS carrier sense indication

to fail to be asserted during a collision on the media. If

the deference process simply times the inter-Frame

gap based on this indication, it is possible for a short in-

terFrame gap to be generated, leading to a potential re-

ception failure of a subsequent frame. To enhance

system robustness, the following optional measures,

as specified in 4.2.8, are recommended when Inter-

Frame-SpacingPart1 is other than 0:

1. Upon completing a transmission, start timing the in-

terrupted gap, as soon as transmitting and carrier

sense are both false.

2. When timing an inter-frame gap following reception,

reset the inter-frame gap timing if carrier sense be-

comes true during the first 2/3 of the inter-frame gap

timing interval. During the final 1/3 of the interval,

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