AM79C90PC datasheet(34/62 Pages) AMD

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P R E L I M I N A R Y

Am79C90

and the CRC gets sampled internally on every byte

boundary. The framing error is reported to the user as

follows:

If the number of the dribbling bits is 1 to 7 bits and

there is no CRC error, then there is no Framing

error (FRAM = 0).

If the number of the dribbling bits is less than 8

and there is a CRC error, then there is also a

Framing error (FRAM = 1).

If the number of the dribbling bits = 0, then there is

no Framing error. There may or may not be a CRC

error.

Interframe Spacing (IFS)

The C-LANCE implements the two-part deferral algo-

rithm following both receive and transmit activity, as

specified as an option in the IEEE 802.3 Standard (ISO/

IEC 8802-3 1990). With two-part deferral, the interframe

spacing, which begins immediately after the negation of

RENA, is divided into two parts, IFS1 and IFS2. If RENA

is asserted during IFS1, the interframe spacing counter

is continually reset until RENA is deasserted (any pend-

ing transmissions will defer to the incoming receive traf-

fic and the incoming frame may be received by the

C-LANCE). Once the interframe spacing counter

reaches IFS2, the counter proceeds, regardless of the

state of RENA. When IFS2 expires, the C-LANCE may

begin transmitting a frame if there is one pending.

In the C-LANCE, IFS1 is 6.0

µs and IFS2 is 3.6 µs, mak-

ing the minimum possible interframe spacing 9.6

µs.

The 9.6

µs minimum interframe spacing complies with

IEEE 802.3 specifications.

Following each frame transmission, the C-LANCE

blinds itself from any receive activity for the first 4.1

µs of

the interframe spacing. The C-LANCE begins looking

for the 011 start frame delimiter pattern after 800ns (8 bit

times) of preamble has passed. Hence, if RENA is as-

serted during the first 4.1

µs of the interframe spacing,

there must be at least 8 bits of preamble left following

the end of the 4.1

µs window in order for the frame to be

received correctly.

Following each frame reception, the C-LANCE blinds it-

self from any receive activity for the first 0.5

µs of the in-

terframe spacing.

Collision Detection and Collision JAM

Collisions are detected by monitoring the CLSN pin. If

CLSN becomes asserted during a frame transmission,

TENA will remain asserted for at least 32 (but not more

than 40) additional bit times (including CLSN synchroni-

zation). This additional transmission after collision is

referred to as COLLISION JAM. If collision occurs

during the transmission of the preamble, the C-LANCE

continues to send the preamble, and sends the JAM pat-

tern following the preamble. If collision occurs after the

preamble, the C-LANCE will send the JAM pattern fol-

lowing the transmission of the current byte. The JAM

pattern is any pattern except the CRC bytes.

Receive Based Collision

If CLSN becomes asserted during the reception of a

packet, this reception is immediately terminated. De-

pending on the timing of COLLISION DETECTION, one

of the following will occur. A collision that occurs within 6

byte times of the detection of the SFD (4.8

µs) will result

in the packet being rejected because of an address mis-

match; the Receive FIFO write pointer will be reset. A

collision that occurs within 64 byte times (51.2

µs) will

result in the packet being rejected since it is a runt pack-

et. A collision that occurs after 64 byte times (late colli-

sion) will result in a truncated packet being written to the

memory buffer with the CRC error bit most likely being

set in the Status Word of the Receive Ring. Late collision

error is not reported in receive mode.

Transmit Based Collision

When a transmission attempt has been terminated due

to the assertion of CLSN, (a collision that occurs within

64 byte times), the C-LANCE will attempt to retry trans-

mission 15 more times. The scheduling of the

retransmissions is determined by a controlled random-

ized process called “truncated binary exponential back-

off.” Upon the negation of the COLLISION JAM interval,

the C-LANCE calculates a delay before retransmitting.

The delay is an integral multiple of the SLOT TIME. The

SLOT TIME is 512 bit times. The number of SLOT

TIMES to delay before the nth retransmission is chosen

as a uniformly distributed random integer in the range:

≤ r ≤ 2k where k = min (n, 10).

When the Modified Backoff Algorithm is enabled

(EMBA), the backoff time may be longer than the mini-

mum time specified above. Specifically, the backoff

count will be suspended whenever a carrier is detected

on the network. The backoff count will resume when the

carrier drops. This behavior has the effect of making the

backoff interval equal to the SUM of an integral number

of SLOT TIMES plus the total duration of the carrier on

the network during the backoff interval.

If all 16 attempts fail, the C-LANCE sets the RTRY bit in

the current Transmit Message Descriptor 3, TMD3, in

memory, gives up ownership (sets the own bit to zero)

for this packet, and processes the next packet in trans-

mit ring for transmission. If there is a late collision (colli-

sion occurring after 64 byte times), the C-LANCE will not

attempt to transmit this packet again; it will terminate the

transmission, note the LCOL error in TMD3, and trans-

mit the next packet in the ring.

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