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參數(shù)資料
| 型號： | NS32490D |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | NIC Network Interface Controller(NIC網(wǎng)絡(luò)接口控制器) |
| 中文描述： | NIC網(wǎng)絡(luò)接口控制器（NIC網(wǎng)絡(luò)接口控制器） |
| 文件頁數(shù)： | 10/56頁 |
| 文件大?。?/td> | 689K |
| 代理商： | NS32490D |
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7.0 Packet Reception
(Continued)
then the packet will essentially be lost and re-transmit-
ted only after a time-out takes place in the upper level
software. By determining that the packet was lost at the
driver level, a transmit command can be reissued to the
NIC once the overflow routine is completed (as in step
11). Also, it is possible for the NIC to defer indefinitely,
when it is stopped on a busy network. Step 5 also allevi-
ates this problem. Step 5 is essential and should not be
omitted from the overflow routine, in order for the NIC to
operate correctly.
6. Place the NIC in either mode 1 or mode 2 loopback. This
can be accomplished by setting bits D2 and D1, of the
Transmit Configuration Register, to ‘‘0,1’’ or ‘‘1,0’’, re-
spectively.
7. Issue the START command to the NIC. This can be ac-
complished by writing 22H to the Command Register.
This is necessary to activate the NIC’s Remote DMA
channel.
8. Remove one or more packets from the receive buffer
ring.
9. Reset the overwrite warning (OVW, overflow) bit in the
Interrupt Status Register.
10. Take the NIC out of loopback. This is done by writing the
Transmit Configuration Register with the value it con-
tains during normal operation. (Bits D2 and D1 should
both be programmed to 0.)
11. If the ‘‘Resend’’ variable is set to a 1, reset the ‘‘Re-
send’’ variable and reissue the transmit command. This
is done by writing a value of 26H to the Command Reg-
ister. If the ‘‘Resend’’ variable is 0, nothing needs to be
done.
Note:
If Remote DMA is not being used, the NIC does not need to be started
before packets can be removed from the receive buffer ring. Hence,
step 8 could be done before step 7.
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 anoth-
er 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
Termination of Received PacketDPacket Accepted
TL/F/8582–10
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.)
BUFFER RECOVERY FOR REJECTED PACKETS
If the packet is a runt packet or contains CRC or Frame
Alignment errors, it is rejected. The buffer management log-
ic 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 PacketDPacket Rejected
TL/F/8582–13
Error Recovery
If the packet is rejected as shown, the DMA is restored by
the NIC by reprogramming the DMA starting address point-
ed to by the Current Page Register.
REMOVING PACKETS FROM THE RING
Packets are removed from the ring using the Remote DMA
or an external device. When using the Remote DMA the
Send Packet command can be used. This programs the Re-
mote DMA to automatically remove the received packet
pointed to by the Boundary Pointer. At the end of the trans-
fer, the NIC moves the Boundary Pointer, freeing additional
buffers for reception. The Boundary Pointer can also be
moved manually by programming the Boundary Register.
Care should be taken to keep the Boundary Pointer at least
one buffer behind the Current Page Pointer.
The following is a suggested method for maintaining the
Receive Buffer Ring pointers.
1. At initialization, set up a software variable (nextDpkt) to
indicate where the next packet will be read. At the begin-
ning of each Remote Read DMA operation, the value of
nextDpkt will be loaded into RSAR0 and RSAR1.
2. When initializing the NIC set:
BNDRY
e
PSTART
CURR
e
PSTART
a
1
nextDpkt
e
PSTART
a
1
10
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