am79c972b Advanced Micro Devices, am79c972b Datasheet - Page 66

• Manufacturer Part Number: am79c972b
• Description: Pcnet?-fast+ Enhanced 10/100 Mbps Pci Ethernet Controller With Onnow Support
• Manufacturer: Advanced Micro Devices

allows simultaneous transmit and receive activity on the
TXDAT and RXDAT pins of the GPSI port, and the
TXD[3:0] and RXD[3:0] pins of the MII port. Full-duplex
operation is enabled by the FDEN bit located in BCR9
for all ports. Full-duplex operation is also enabled
through Auto-Negotiation when DANAS (BCR 32, bit 7)
is not enabled on the MII port and the ASEL bit is set,
and both the external PHY and its link partner are ca-
pable of Auto-Negotiation and full-duplex operation.
When operating in full-duplex mode, the following
changes to the device operation are made:
Bus Interface/Buffer Management Unit changes:
n The first 64 bytes of every transmit frame are not
n Successful reception of the first 64 bytes of every
The MAC engine changes for full-duplex operation are
as follows:
66
preserved in the Transmit FIFO during transmission
of the first 512 bits as described in the Transmit Ex-
ception Conditions section. Instead, when full-du-
plex mode is active and a frame is being transmitted,
the XMTFW bits (CSR80, bits 9-8) always govern
when transmit DMA is requested.
receive frame is not a requirement for Receive DMA
to begin as described in the Receive Exception Con-
ditions section. Instead, receive DMA will be re-
quested as soon as either the RCVFW threshold
(CSR80, bits 12-13) is reached or a complete valid
receive frame is detected, regardless of length. This
Receive FIFO operation is identical to when the RPA
bit (CSR124, bit 3) is set during half-duplex mode
operation.
Am79C972
n Changes to the Transmit Deferral mechanism:
n The 4.0 µs carrier sense blinding period after a
n The collision indication input to the MAC engine is
The MII changes for full-duplex operation are as fol-
lows:
n The collision detect (COL) pin is disabled.
n The SQE test function is disabled.
n Loss of Carrier (LCAR) reporting is disabled.
Full-Duplex Link Status LED Support
The Am79C972 controller provides bits in each of the
LED Status registers (BCR4, BCR5, BCR6, BCR7) to
display the Full-Duplex Link Status. If the FDLSE bit (bit
8) is set, a value of 1 will be sent to the associated LED-
OUT bit when in Full-Duplex.
— Transmission is not deferred while receive is
— The IPG counter which governs transmit deferral
transmission during which the SQE test normally
occurs is disabled.
ignored.
active.
during the IPG between back-to-back transmits
is started when transmit activity for the first
packet ends, instead of when transmit and car-
rier activity ends.