am79c960 Advanced Micro Devices, am79c960 Datasheet - Page 56

• Manufacturer Part Number: am79c960
• Description: Pcnettm-isa Single-chip Ethernet Controller
• Manufacturer: Advanced Micro Devices

the completion of transmission, then the PCnet-ISA
controller will set the CERR bit in CSR0. CERR will be
asserted in 10BASE-T mode after transmit if T-MAU is
in Link Fail state. CERR will never cause INTR to be acti-
vated. It will, however, set the ERR bit in CSR0.
Host related transmit exception conditions include
BUFF and UFLO as described in the Transmit Descrip-
tor section.
Receive Operation
The receive operation and features of the PCnet-ISA
controller are controlled by programmable options.
Receive Function Programming
Automatic pad field stripping is enabled by setting the
ASTRP_RCV bit in CSR4; this can provide flexibility in
the reception of messages using the 802.3 frame
format.
All receive frames can be accepted by setting the PROM
bit in CSR15. When PROM is set, the PCnet-ISA con-
troller will attempt to receive all messages, subject to
minimum frame enforcement. Promiscuous mode over-
rides the effect of the Disable Receive Broadcast bit on
receiving broadcast frames.
The point at which the BMU will start to transfer data
from the receive FIFO to buffer memory is controlled by
the RCVFW bits in CSR80. The default established dur-
ing reset is 10b, which sets the threshold flag at 64 bytes
empty.
1-398
AMD
1010....1010
Preamble
Bits
56
Increasing Time
Start of Packet
at Time= 0
10101011
SYNCH
Bits
802.3 Frame and Length Field Transmission Order
8
ADDR
Bytes
Dest
6
P R E L I M I N A R Y
Am79C960
Bit
ADDR
0
Bytes
Srce
Significant
6
Most
Byte
Automatic Pad Stripping
During reception of an 802.3 frame the pad field can be
stripped automatically. ASTRP_RCV (bit 10 in CSR4) =
1 enables the automatic pad stripping feature. The pad
field will be stripped before the frame is passed to the
FIFO, thus preserving FIFO space for additional frames.
The FCS field will also be stripped, since it is computed
at the transmitting station based on the data and pad
field characters, and will be invalid for a receive frame
that has had the pad characters stripped.
The number of bytes to be stripped is calculated from
the embedded length field (as defined in the IEEE 802.3
definition) contained in the frame. The length indicates
the actual number of LLC data bytes contained in the
message. Any received frame which contains a length
field less than 46 bytes will have the pad field stripped (if
ASTRP_RCV is set). Receive frames which have a
length field of 46 bytes or greater will be passed to the
host unmodified.
Since any valid Ethernet Type field value will always be
greater than a normal 802.3 Length field ( 46), the
PCnet-ISA controller will not attempt to strip valid Ether-
net frames.
Note that for some network protocols the value passed
in the Ethernet Type and/or 802.3 Length field is not
compliant with either standard and may cause
problems.
The diagram below shows the byte/bit ordering of the re-
ceived length field for an 802.3 compatible frame format.
Bit
Length
Bytes
7
2
Bit
0
Significant
Least
Byte
1–1500
Bytes
DATA
LLC
Bit
7
46–1500
Bytes
16907B-13
Bytes
45–0
Pad
Bytes
FCS
4