~ National ~ Semiconductor - Al Kossow's Bitsavers

7.0 Packet Reception (Continued)
Received Packet Aborted If It Hits Boundary Pointer
Buffer Ring Overflow
TL/F/9345-12
If the Buffer Ring has been filled and the DMA reaches the
Boundary Pointer Address, reception of the incoming packet
will be aborted by the NIC. Thus, the packets previously
received and still contained in the Ring will not be destroyed.
In a heavily loaded network environment the local DMA may
be disabled, preventing the NIC from buffering packets from
the network. To guarantee this will not happen, a software
reset must be issued during all Receive Buffer Ring overflows
(indicated by the OVW bit in the Interrupt Status Register).
The following procedure is required to recover
from a Receiver Buffer Ring Overflow.
1. Issue the STOP mode command (Command Register =
21 H). The NIC may not immediately enter the STOP
mode. If it is currently processing a packet, the NIC will
enter STOP mode only after finishing the packet. The NIC
indicates that it has entered STOP mode by setting the
RST bit in the Interrupt Status Register.
2. Clear the Remote Byte Counter Registers (RBCRO,
RBCR1). The NIC requires these registers to be cleared
before it sets the RST bit.
Note: If the STP is set when a transmission is in progress, the RST bit may
not be set. In this case, the NIC is guaranteed to be reset after the
longest packet time (1500 bytes ~ 1.2 ms). For the DP8390C (but not
for the DP8390B), the NIC will be reset wtthin 2 microseconds after
the STP bit is set and Loopback mode 1 is programmed.
3. Poll the Interrupt Status Register for the RST bit. When
set, the NIC is in STOP mode.
4. Place the NIC in LOOPBACK (mode 1 or 2) by writing
02H or 04H to the Transmit Configuration Register. This
step is required to properly enable the NIC onto an active
network.
5. Issue the START mode command (Command Register =
22H). The local receive DMA is still inactive since the NIC
is in LOOPBACK.
6. Remove at least one packet from the Receive Buffer
Ring to accommodate additional incoming packets.
7. Take the NIC out of LOOPBACK by programming the
Transmit Configuration Register back to its original value
and resume normal operation.
Note: If the Remote DMA channel is not used, you may eliminate step 6 and
remove packets from the Receive Buffer Ring after step 1. This will
reduce or eliminate the polling time incurred in step 3.
END OF PACKET OPERATIONS
At the end of the packet the NIC determines whether the
received packet is to be accepted or rejected. It either
branches to a routine to store the Buffer Header or to another
routine that recovers the buffers used to store the packet.
SUCCESSFUL RECEPTION
If the packet is successfully received as shown, the DMA is
restored to the first buffer used to store the packet (pointed
to by the Current Page Register). The DMA then stores the
Receive Status, a Pointer to where the next packet will be
stored (Buffer 4) and the number of received bytes. Note
that the remaining bytes in the last buffer are discarded and
reception of the next packet begins on the next empty 256-
byte buffer boundary. The Current Page Register is then
initialized to the next available buffer in the Buffer Ring. (The
location of the next buffer had been previously calculated
and temporarily stored in an internal scratchpad register.)
Termination of Received Packet-Packet Accepted
BUFFER RECOVERY FOR REJECTED PACKETS
TL/F/9345-13
If the packet is a runt packet or contains CRC or Frame
Alignment errors, it is rejected. The buffer management logic
resets the DMA back to the first buffer page used to store
the packet (pointed to by CURR), recovering all buffers that
had been used to store the rejected packet. This operation
will not be performed if the NIC is programmed to accept
either runt packets or packets with CRC or Frame Alignment
errors. The received CRC is always stored in buffer memory
after the last byte of received data for the packet.
Termination of Received Packet-Packet Rejected
TLlF/9345-14
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