am79c960 Advanced Micro Devices, am79c960 Datasheet - Page 57

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

Receive FCS Checking
Reception and checking of the received FCS is per-
formed automatically by the PCnet-ISA controller. Note
that if the Automatic Pad Stripping feature is enabled,
the received FCS will be verified against the value com-
puted for the incoming bit stream including pad
characters, but it will not be passed to the host. If a FCS
error is detected, this will be reported by the CRC bit in
RMD1.
Receive Exception Conditions
Exception conditions for frame reception fall into two
distinct categories; those which are the result of normal
network operation, and those which occur due to abnor-
mal network and/or host related events.
Normal events which may occur and which are handled
autonomously by the PCnet-ISA controller are basically
collisions within the slot time and automatic runt packet
rejection. The PCnet-ISA controller will ensure that colli-
sions which occur within 512 bit times from the start of
reception (excluding preamble) will be automatically de-
leted from the receive FIFO with no host intervention.
The receive FIFO will delete any frame which is com-
posed of fewer than 64 bytes provided that the Runt
Packet Accept (RPA bit in CSR124) feature has not
been enabled. This criteria will be met regardless of
whether the receive frame was the first (or only) frame in
the FIFO or if the receive frame was queued behind a
previously received message.
Abnormal network conditions include:
These should not occur on a correctly configured 802.3
network and will be reported if they do.
Host related receive exception conditions include MISS,
BUFF, and OFLO. These are described in the Receive
Descriptor section.
Loopback Operation
During loopback, the FCS logic can be allocated to the
receiver by setting the DXMTFCS bit in CSR15.
If DXMTFCS=0, the MAC Engine will calculate and ap-
pend the FCS to the transmitted message. In this
loopback configuration, the receive circuitry cannot de-
tect FCS errors if they occur.
If DXMTFCS=1, the last four bytes of the transmit mes-
sage must contain the (software generated) FCS
computed for the transmit data preceding it. The MAC
Engine will transmit the data without addition of an FCS
field, and the FCS will be calculated and verified at the
receiver.
FCS errors
Late collision
P R E L I M I N A R Y
Am79C960
The loopback facilities of the MAC Engine allow full op-
eration to be verified without disturbance to the network.
Loopback operation is also affected by the state of the
Loopback Control bits (LOOP, MENDECL, and INTL) in
CSR15. This affects whether the internal MENDEC is
considered part of the internal or external loopback
path.
When in the loopback mode(s), the multicast address
detection feature of the MAC Engine, programmed by
the contents of the Logical Address Filter (LADRF [63:0]
in CSR 8-11) can only be tested when DXMTFCS= 1, al-
locating the FCS generator to the receiver. All other
features operate identically in loopback as in normal op-
eration, such as automatic transmit padding and receive
pad stripping.
When performing an internal loopback, no frame will be
transmitted to the network. However, when the
PCnet-ISA controller is configured for internal loopback
the receiver will not be able to detect network traffic. AUI
external loopback tests will transmit frames onto the net-
work, and the PCnet-ISA controller will receive network
traffic while configured for external loopback. 10BASE-
T external loopback should never be used in a live
network. 10BASE-T external loopback provides a
means of looping Transmit data to the receive input
without asserting a collision. This mode allows a board
test to verify both the transmit and receive paths to the
10BASE-T connector. Unless the Runt Packet Accept
feature is enabled, all loopback frames must contain at
least 64 bytes of data.
LEDs
The PCnet-ISA controller’s LED control logic allows pro-
gramming of the status signals, which are displayed on
3 LED outputs. One LED (LED0) is dedicated to display-
ing 10BASE-T Link Status. The status signals available
are Collision, Jabber, Receive, Receive Polarity (active
when receive polarity is okay), and Transmit. If more
than one status signal is enabled, they are ORed to-
gether. An optional pulse stretcher is available for each
programmable output. This allows emulation of the
TPEX (Am79C98) and TPEX+ (Am79C100) LED
outputs.
RVPOL
LNKST
Signal
RCV
XMT
Active during Link OK
Not active during Link Down
Active while receiving data
Active during receive polarity is OK
Not active during reverse receive polarity
Active while transmitting data
Behavior
AMD
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