AM79C970A datasheet(73/219 Pages) AMD

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P R E L I M I N A R Y

AMD

Am79C970A

Error Detection

The MAC engine provides several facilities which report

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 following transmit

status is available in the appropriate Transmit Message

Descriptor (TMD) and Control and Status Register

(CSR) areas:

s The number of transmission retry attempts (ONE,

MORE, RTRY, and TRC).

s Whether the MAC engine had to Defer (DEF) due to

channel activity.

s Excessive deferral (EXDEF), indicating that the

transmitter has experienced Excessive Deferral on

this transmit frame, where Excessive Deferral is de-

fined in ISO 8802-3 (IEEE/ANSI 802.3).

s Loss of Carrier (LCAR), indicating that there was an

interruption in the ability of the MAC engine to

monitor its own transmission. Repeated LCAR er-

rors indicate a potentially faulty transceiver or net-

work connection.

s Late Collision (LCOL) indicates that the transmission

suffered a collision after the slot time. This is indica-

tive of a badly configured network. Late collisions

should not occur in a normal operating network.

s Collision Error (CERR) indicates that the transceiver

did not respond with an SQE Test message within

the first 4

µs after a transmission was completed.

This

may

due

failed

transceiver,

disconnected or faulty transceiver drop cable, or the

fact the transceiver does not support this feature (or

it is disabled).

In addition to the reporting of network errors, the MAC

engine will also attempt 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 un-

derflow, the MAC engine will guarantee the message is

either sent as a runt packet (which will be deleted by the

receiving station) or has an invalid FCS (which will also

cause the receiver to reject the message).

The status of each receive message is available in the

appropriate Receive Message Descriptor (RMD) and

CSR areas. All received frames are passed to the host

regardless of any error. The FRAM error will only be re-

ported if an FCS error is detected and there are a non

integral number of bytes in the message.

During the reception, the FCS is generated on every se-

rial bit (including the dribbling bits) coming from the ca-

ble, although the internally saved FCS value is only

updated on the eighth bit (on each byte boundary). The

MAC engine will ignore up to 7 additional bits at the end

of a message (dribbling bits), which can occur under

normal network operating conditions. The framing error

is reported to the user as follows:

s If the number of dribbling bits are 1 to 7 and there is

no FCS error, then there is no Framing error

(FRAM = 0).

s If the number of dribbling bits are 1 to 7 and there is a

FCS error, then there is also a Framing error

(FRAM = 1).

s If the number of dribbling bits is ZERO, then there is

no Framing error. There may or may not be a

FCS error.

s If the number of dribbling bits is EIGHT, then there is

no Framing error. FCS error will be reported and the

receive message count will indicated one extra byte.

Counters are provided to report the Receive Collision

Count and Runt Packet Count, for network statistics and

utilization calculations.

Note that if the MAC engine detects a received frame

which has a 00b pattern in the preamble (after the first

8-bits which are ignored), the entire frame will be ig-

nored. The MAC engine will wait for the network to go

inactive before attempting to receive additional frames.

Media Access Management

The basic requirement for all stations on the network is

provide

fairness

channel

allocation.

The

802.3/Ethernet protocols define a media access mecha-

nism 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 Pack-

et Gap) after the last activity, before transmitting on the

media. The channel is a multidrop communications me-

dia (with various topological configurations permitted)

which allows a single station to transmit and all other

stations to receive. If two nodes simultaneously contend

for the channel, their signals will interact causing loss of

data, defined as a collision. It is the responsibility of the

MAC to attempt to avoid and recover from a collision, to

guarantee data integrity for the end-to-end transmission

to the receiving station.

Medium Allocation

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 also al-

lows optional two part deferral after a receive message.

See ANSI/IEEE Std 802.3-1990 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 interFrame gap

based on this indication it is possible for a short

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