AM79C976KI AMD (ADVANCED MICRO DEVICES), AM79C976KI Datasheet - Page 74

• Manufacturer Part Number: AM79C976KI
• Manufacturer: AMD (ADVANCED MICRO DEVICES)

Specifications of AM79C976KI

• Operating Supply Voltage (typ): 3.3V
• Operating Supply Voltage (min): 2.97V
• Operating Supply Voltage (max): 3.63V
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 208
• Lead Free Status / RoHS Status: Not Compliant

always accepted, except when DRCVBC (CSR15, bit
14) is set. DRCVBC overrides a logical address match.
If DRCVBC is set to 1, broadcast frames are not ac-
cepted even if the Logical Address Filter is pro-
grammed in such a way that a Broadcast frame would
pass the hash filter.
None of the address filtering described above applies
when the Am79C976 controller is operating in the pro-
miscuous mode. In the promiscuous mode, all properly
formed packets are received, regardless of the con-
tents of their destination address fields. The promiscu-
ous mode overrides the Disable Receive Broadcast bit
(DRCVBC bit Am79C976 in the MODE register) and
the Disable Receive Physical Address bit (DRCVPA,
CSR15, bit 13).
The Am79C976 controller operates in promiscuous
mode when PROM (CSR15, bit 15) is set.
In addition, the Am79C976 controller provides the Ex-
ternal Address Detection Interface (EADI) to allow ex-
ternal address filtering. See the External Address
Detection Interface section for further details.
The receive descriptor entry RMD1 contains three bits
that indicate which method of address matching
caused the Am79C976 controller to accept the frame.
Note that these indicator bits are not available when
the Am79C976 controller is programmed to use 16-bit
structures for the descriptor entries (BCR20, bit 7-0,
SWSTYLE is set to 0).
PAM (RMD1, bit 22) is set by the Am79C976 controller
when it accepts the received frame due to a match of
the frame’s destination address with the content of the
physical address register.
LAFM (RMD1, bit 21) is set by the Am79C976 control-
ler when it accepts the received frame based on the
value in the logical address filter register.
BAM (RMD1, bit 20) is set by the Am79C976 controller
when it accepts the received frame because the
frame’s destination address is of the type ’Broadcast’.
Only BAM, but not LAFM, will be set when a Broadcast
frame is received, even if the Logical Address Filter is
programmed in such a way that a Broadcast frame
would pass the hash filter.
When the Am79C976 controller operates in promiscu-
ous mode and none of the three match bits is set, it is
74
P R E L I M I N A R Y
Am79C976
an indication that the Am79C976 controller only ac-
cepted the frame because it was in promiscuous mode.
When the Am79C976 controller is not programmed to
be in promiscuous mode, but the EADI interface is
used and when none of the three match bits is set, it is
an indication that the Am79C976 controller only ac-
cepted the frame because it was not rejected by driving
the EAR pin LOW during the receive protect time. The
length of receive protect period can be programmed in
the Receive Protect Register.
See Table 6 for receive address matches.
Automatic Pad Stripping
During reception of an IEEE 802.3 frame, the pad field
can be stripped automatically. Setting ASTRP_RCV
(CSR4, bit 10) to 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.
Figure 31 shows the byte/bit ordering of the received
length field for an IEEE 802.3-compatible frame format.
PAM
0
1
0
0
Table 6. Receive Address Match
LAFM
0
0
1
0
BAM
0
0
0
1
Frame accepted due to
PROM = 1 or no EADI
reject
Physical address match
Logical address filter
match;
frame is not of type
broadcast
Broadcast frame
Comment
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