AM79C90 Advanced Micro Devices, AM79C90 Datasheet - Page 34

• Manufacturer Part Number: AM79C90
• Description: CMOS Local Area Network Controller for Ethernet (C-LANCE)
• Manufacturer: Advanced Micro Devices

and the CRC gets sampled internally on every byte
boundary. The framing error is reported to the user as
follows:
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 s and IFS2 is 3.6 s, mak-
ing the minimum possible interframe spacing 9.6 s.
The 9.6 s 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 s 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 s of the interframe spacing,
there must be at least 8 bits of preamble left following
the end of the 4.1 s 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 s 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
34
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.
AMD
P R E L I M I N A R Y
Am79C90
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 s) 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 s) 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:
0 r
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.
2
k
where k = min (n, 10).