AM79C90 Advanced Micro Devices, AM79C90 Datasheet - Page 24

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

Bit
15
14:08
07
06
05
24
AMD
PROM
EMBA
Name
DRTY
INTL
RES
Description
PROMISCUOUS
PROM = 1, all incoming packets are
accepted.
RESERVED. Read as zeroes. Write
as zeroes.
Enable Modified Back-off Algorithm.
When set (EMBA=1), enables the
modified backoff algorithm. EMBA
is cleared by activation of the RESET
pin or setting the STOP bit.
INTERNAL LOOPBACK is used with
the LOOP bit to determine where the
loopback is to be done. Internal loop-
back allows the chip to receive its
own transmitted packet. Since this
represents full duplex operation, the
packet size is limited to 8–32 bytes.
Internal loopback in the C-LANCE is
operational when the packets are
addressed to the node itself.
The C-LANCE will not receive any
packets externally when it is in inter-
nal loopback mode.
EXTERNAL LOOPBACK allows the
C-LANCE to transmit a packet
through the SIA transceiver cable
out to the Ethernet medium. It is
used to determine the operability of
all circuitry and connections be-
tween the C-LANCE and the physi-
cal medium. Multicast addressing in
external loopback is valid only when
DTCR = 1 (user needs to append the
4 bytes CRC).
In external loopback, the C-LANCE
also receives packets from other
nodes. The FIFOs READ/WRITE
pointers
C-LANCE under heavy traffic. The
packet could then be corrupted or
not received. Therefore, the external
loopback execution may need to be
repeated. See specific discussion
under “Loopback” in later section.
INTL is only valid if LOOP = 1; other-
wise, it is ignored.
DISABLE RETRY. When DRTY = 1,
the C-LANCE will attempt only one
transmission of a packet. If there is a
collision on the first transmission at-
tempt, a Retry Error (RTRY) will be
reported in Transmit Message De-
scriptor 3 (TMD
LOOP
0
1
1
may
INTL
X
0
1
3
).
misalign
mode.
LOOPBACK
No loopback,
normal
External
Internal
P R E L I M I N A R Y
in
When
the
Am79C90
Bit
04
03
02
Name
DTCR
LOOP
COLL
Description
FORCE COLLISION. This bit allows
the collision logic to be tested. The
C-LANCE must be in internal loop-
back mode for COLL to be valid. If
COLL = 1, a collision will be forced
during the subsequent transmission
attempt. This will result in 16 total
transmission attempts with a retry er-
ror reported in TMD
DISABLE TRANSMIT CRC. When
DTCR = 0, the transmitter will gener-
ate and append a CRC to the trans-
mitted packet. When DTCR = 1, the
CRC logic is allocated to the receiver
and no CRC is generated and sent
with the transmitted packet. The
ADD_FCS bit (bit 13, TMD
used to override a DTCR=1 setting
on a per packet basis.
During loopback, DTCR = 0 will
cause a CRC to be generated on the
transmitted packet, but no CRC
check will be done by the receiver
since the CRC logic is shared and
cannot generate and check CRC at
the same time. The generated CRC
will be written into memory with the
data and can be checked by the host
software.
If DTCR = 1 during loopback, the
host software must append a CRC
value to the transmit data.
The receiver will check the CRC on
the received data and report any
errors.
LOOPBACK allows the C-LANCE to
operate in full duplex mode for test
purposes. The packet size is limited
to 8–32 bytes.The received packet
can be up to 36 bytes (32 + 4 bytes
CRC) when DTCR = 0. During loop-
back, the runt packet filter is disabled
because the maximum packet is
forced to be smaller than the
minimum
(64 bytes).
LOOP = 1 allows simultaneous
transmission and reception for a
message constrained to fit within the
Transmit FIFO. The C-LANCE waits
until the entire message is in the
Transmit FIFO before serial trans-
mission begins. The incoming data
stream fills the Receive FIFO. Mov-
ing the received message out of the
Receive FIFO to memory does not
begin until reception has ceased.
size
Ethernet
3
.
1
) can be
packet