AM79C973BKD AMD (ADVANCED MICRO DEVICES), AM79C973BKD Datasheet - Page 89

• Manufacturer Part Number: AM79C973BKD
• Manufacturer: AMD (ADVANCED MICRO DEVICES)

Specifications of AM79C973BKD

• Lead Free Status / RoHS Status: Compliant

ler, unless the EAR pin becomes active during the first
64 bytes of the frame (excluding preamble and SFD).
This allows external address lookup logic approxi-
mately 58 byte times after the last destination address
bit is available to generate the EAR signal, assuming
that the Am79C973/Am79C975 controller is not config-
ured to accept runt packets. The EADI logic only sam-
ples EAR from 2 bit times after SFD until 512 bit times
(64 bytes) after SFD. The frame will be accepted if EAR
has not been asserted during this window. In order for
the EAR pin to be functional in full-duplex mode, FDR-
PAD bit (BCR9, bit 2) needs to be set. If Runt Packet
Accept (CSR124, bit 3) is enabled, then the EAR signal
must be generated prior to the 8 bytes received, if
frame rejection is to be guaranteed. Runt packet sizes
could be as short as 12 byte times (assuming 6 bytes
for source address, 2 bytes for length, no data, 4 bytes
for FCS) after the last bit of the destination address is
available. EAR must have a pulse width of at least 110
ns.
The EADI outputs continue to provide data throughout
the reception of a frame. This allows the external logic
to capture frame header information to determine pro-
tocol type, internetworking information, and other use-
ful data.
The EADI interface will operate as long as the STRT bit
in CSR0 is set, even if the receiver and/or transmitter
are disabled by software (DTX and DRX bits in CSR15
are set). This configuration is useful as a semi-power-
down mode in that the Am79C973/Am79C975 control-
ler will not perform any power-consuming DMA opera-
tions. However, external circuitry can still respond to
control frames on the network to facilitate remote node
control. Table 11 summarizes the operation of the EADI
interface.
External Address Detection Interface: MII Snoop
Mode
The MII Snoop mode provides all necessary data and
clock signals needed for the EADI interface. Data for
the EADI is the RXD[3:0] receive data provided to the
internal MII. The user will receive the data as 4 bit nib-
bles. RX_CLK is provided to allow clocking of the
RXD[3:0] receive nibble stream into the external ad-
dress detection logic. The RXD[3:0] data is synchro-
nous to the rising edge of the RX_CLK. The data
PROM
1
0
0
EAR
X
1
0
Table 11. EADI Operations
No timing
requirements
No timing
requirements
Low for two bit
times plus 10 ns
Required
Timing
All received frames
All received frames
Frame rejected if in
address match
mode
Received
Frames
P R E L I M I N A R Y
Am79C973/Am79C975
arrives in nibbles and can be at a rate of 25 MHz or 2.5
MHz.
The assertion of SFBD is a signal to the external ad-
dress detection logic that the SFD has been detected
and that the first valid data nibble is on the RXD[3:0]
data bus. The SFBD signal is delayed one RX_CLK
cycle from the above definition and actually signals the
start of valid data. In order to reduce the amount of
logic external to the Am79C973/Am79C975 controller
for multiple address decoding systems, the SFBD sig-
nal will go HIGH at each new byte boundary within the
packet, subsequent to the SFD. This eliminates the
need for externally supplying byte framing logic.
The EAR pin should be driven LOW by the external ad-
dress comparison logic to reject a frame.
External Address Detection Interface: Receive
Frame Tagging
The Am79C973/Am79C975 controller supports re-
ceive frame tagging in MII Snoop mode. The receive
frame tagging implementation is a two-wire chip inter-
face in addition to the existing EADI.
The Am79C973/Am79C975 controller supports up to
15 bits of receive frame tagging per frame in the receive
frame status (RFRTAG). The RFRTAG bits are in the
receive frame status field in RMD2 (bits 30-16) in 32-bit
software mode. The receive frame tagging is not sup-
ported in the 16-bit software mode. The RFRTAG field
are all zeros when either the EADISEL (BCR2, bit3) or
the RXFRTAG (CSR7, bit 14) are set to 0. When
EADISEL (BCR2, bit 3) and RXFRTAG (CSR7, bit 14)
are set to 1, then the RFRTAG reflects the tag word
shifted in during that receive frame.
In the MII Snoop mode, the two-wire interface will use
the MIIRXFRTGD and MIIRXFRTGE pins from the
EADI interface. These pins will provide the data input
and data input enable for the receive frame tagging, re-
spectively. These pins are normally not used during the
MII operation.
The receive frame tag register is a shift register that
shifts data in MSB first, so that less than the 15 bits al-
located may be utilized by the user. The upper bits not
utilized will return zeros. The receive frame tag register
is set to 0 in between reception of frames. After receiv-
ing SFBD indication on the EADI, the user can start
shifting data into the receive tag register until one net-
work clock period before the Am79C973/Am79C975
controller receives the end of the current receive frame.
In the MII Snoop mode, the user must see the RX_CLK
to drive the synchronous receive frame tag data inter-
face. After receiving the SFBD indication, sampled by
the rising edge of the RX_CLK, the user will drive the
data input and the data input enable synchronous with
the rising edge of the RX_CLK. The user has until one
network clock period before the deassertion of the
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