AM79C971VCW Advanced Micro Devices, AM79C971VCW Datasheet - Page 63

• Manufacturer Part Number: AM79C971VCW
• Description: PCnet-FAST Single-Chip Full-Duplex 10/100 Mbps Ethernet Controller for PCI Local Bus
• Manufacturer: Advanced Micro Devices

See ANSI/IEEE Std 802.3-1993 Edition, 7.2.4.6 (1):
The Am79C971 controller implements a carrier sense
“blinding” period of 4.0 s length starting from the
deassertion of carrier sense after transmission. This ef-
fectively means that when transmit two part deferral is
enabled (DXMT2PD is cleared), the IFS1 time is from
4 s to 6 s after a transmission. However, since IPG
shrinkage below 4 s will rarely be encountered on a
correctly configured network, and since the fragment
size will be larger than the 4 s blinding window, the
IPG counter will be reset by a worst case IPG shrink-
age/fragment scenario and the Am79C971 controller
will defer its transmission. If carrier is detected within
the 4.0 to 6.0 s IFS1 period, the Am79C971 controller
will not restart the “blinding” period, but only restart
IFS1.
Collision Handling
Collision detection is performed and reported to the
MAC engine by the integrated Manchester Encoder/
Decoder (MENDEC) and through the MII via the COL
input pin. Both are functionally equivalent in operation.
If a collision is detected before the complete preamble/
SFD sequence has been transmitted, the MAC engine
will complete the preamble/SFD before appending the
jam sequence. If a collision is detected after the pream-
ble/SFD has been completed, but prior to 512 bits
being transmitted, the MAC engine will abort the trans-
mission and append the jam sequence immediately.
The jam sequence is a 32-bit all zeros pattern.
The MAC engine will attempt to transmit a frame a total
of 16 times (initial attempt plus 15 retries) due to nor-
mal collisions (those within the slot time). Detection of
collision will cause the transmission to be rescheduled
to a time determined by the random backoff algorithm.
If a single retry was required, the 1 bit will be set in the
transmit frame status. If more than one retry was re-
quired, the MORE bit will be set. If all 16 attempts ex-
perienced collisions, the RTRY bit will be set (1 and
MORE will be clear), and the transmit message will be
flushed from the FIFO. If retries have been disabled by
setting the DRTY bit in CSR15, the MAC engine will
abandon transmission of the frame on detection of the
first collision. In this case, only the RTRY bit will be set
“At the conclusion of the output function, the DTE
opens a time window during which it expects to see
the signal_quality_error signal asserted on the
Control In circuit. The time window begins when
the
CARRIER_OFF. If execution of the output function
does not cause CARRIER_ON to occur, no SQE
test occurs in the DTE. The duration of the window
shall be at least 4.0 s but no more than 8.0 s.
During the time window the Carrier Sense Function
is inhibited.”
CARRIER_STATUS
becomes
Am79C971
and the transmit message will be flushed from the
FIFO.
If a collision is detected after 512 bit times have been
transmitted, the collision is termed a late collision. The
MAC engine will abort the transmission, append the
jam sequence, and set the LCOL bit. No retry attempt
will be scheduled on detection of a late collision, and
the transmit message will be flushed from the FIFO.
The ISO 8802-3 (IEEE/ANSI 802.3) Standard requires
use of a “truncated binary exponential backoff” algo-
rithm, which provides a controlled pseudo random
mechanism to enforce the collision backoff interval,
before retransmission is attempted.
See ANSI/IEEE Std 802.3-1990 Edition, 4.2.3.2.5:
The Am79C971 controller provides an alternative algo-
rithm, which suspends the counting of the slot time/IPG
during the time that receive carrier sense is detected.
This aids in networks where large numbers of nodes
are present, and numerous nodes can be in collision. It
effectively accelerates the increase in the backoff time
in busy networks and allows nodes not involved in the
collision to access the channel, while the colliding
nodes await a reduction in channel activity. Once chan-
nel activity is reduced, the nodes resolving the collision
time-out their slot time counters as normal.
This modified backoff algorithm is enabled when EMBA
(CSR3, bit 3) is set to 1.
Transmit Operation
The transmit operation and features of the Am79C971
controller are controlled by programmable options. The
Am79C971 controller offers a large transmit FIFO to
provide frame buffering for increased system latency,
automatic retransmission with no FIFO reload, and au-
tomatic transmit padding.
Transmit Function Programming
Automatic transmit features such as retry on collision,
FCS generation/transmission, and pad field insertion
can all be programmed to provide flexibility in the (re-)
transmission of messages.
Disable retry on collision (DRTY) is controlled by the
DRTY bit of the Mode register (CSR15) in the initializa-
tion block.
Automatic pad field insertion is controlled by the
APAD_XMT bit in CSR4.
“At the end of enforcing a collision (jamming), the
CSMA/CD sublayer delays before attempting to re-
transmit the frame. The delay is an integer multiple
of slot time. The number of slot times to delay be-
fore the nth retransmission attempt is chosen as a
uniformly distributed random integer r in the range:
0
r < 2
k
where k = min (n,10).”
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