am79c970 Advanced Micro Devices, am79c970 Datasheet - Page 73

• Manufacturer Part Number: am79c970
• Description: Pcnettm-pci Single-chip Ethernet Controller For Pci Local Bus
• Manufacturer: Advanced Micro Devices

goes inactive (this does not apply if the 10BASE-T port
is selected). If the CI input is not asserted within the 40
network bit time period following the completion of
transmission, then the PCnet-PCI 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 INTA to be activated. It will, however,
set the ERR bit CSR0.
Receive Operation
The receive operation and features of the PCnet-PCI
controller are controlled by programmable options.
Address Matching
The PCnet-PCI controller supports three types of ad-
dress matching: unicast, multicast, and broadcast. The
normal address matching procedure can be modified by
programming three bits in the MODE register (PROM,
DRCVBC, and DRCBC).
If the first bit received after the start of frame delimiter
(the least significant bit of the first byte of the destination
address field) is 0, the frame is unicast, which indicates
that the frame is meant to be received by a single node.
If the first bit received is 1, the frame is multicast, which
indicates that the frame is meant to be received by a
group of nodes. If the destination address field contains
all ones, the frame is broadcast, which is a special type
of multicast. Frames with the broadcast address in the
destination address field are meant to be received by all
nodes on the local area network.
When a unicast frame arrives at the PCnet-PCI control-
ler, the controller will accept the frame if the destination
address field of the incoming frame exactly matches the
6-byte station address stored in the PADR registers
(CSR12, CSR13, and CSR14). The byte ordering is
such that the first byte received from the network (after
the SFD) must match the least significant byte of CSR12
(PADR[7:0]), and the sixth byte received must match the
most significant byte of CSR14 (PADR[47:40]).
If DRCVPA (bit 13 in the MODE register) is set. the
PCnet-PCI controller will not accept unicast frames.
If the incoming frame is multicast the PCnet-PCI control-
ler performs a calculation on the contents of the destina-
tion address field to determine whether or not to accept
the frame. This calculation is explained in the section
that describes the Logical Address Filter (LADRF).
If all bits of the LADRF registers are 0 no multicast
frames are accepted, except for broadcast frames.
Although broadcast frames are classified as special
multicast frames, they are treated differently by the
PCnet-PCI controller hardware. Broadcast frames are
always accepted, except when DRCVBC (bit 14 in the
MODE register) is set.
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AMD
P R E L I M I N A R Y
Am79C970
None of the address filtering described above applies
when the PCnet-PCI controller is operating in the pro-
miscuous mode. In the promiscuous mode, all properly
formed packets are received, regardless of the contents
of their destination address fields. The promiscuous
mode overrides the Disable Receive Broadcast bit
(DRCVBC bit l4 in the MODE register) and the Disable
Receive Physical Address bit (DRCVPA, bit 13 MODE
register).
The PCnet-PCI controller operates in promiscuous
mode when PROM (bit 15 in the MODE register) is set.
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-PCI 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 H_RESET is 10b which sets the threshold flag at
64 bytes empty.
Automatic Pad Stripping
During reception of an 802.3 frame the pad field can be
stripped automatically. ASTRP_RCV (CSR4, bit 0) = 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 ISO 8802-3
(IEEE/ANSI 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-PCI controller will not attempt to strip valid Ether-
net frames.