DP8390D datasheet(10/56 Pages) NSC

10 / 56 page

70 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 ‘‘01’’ or ‘‘10’’ 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 PacketPacket Accepted

TLF8582 – 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 PacketPacket Rejected

TLF8582 – 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 (next

pkt) to

indicate where the next packet will be read At the begin-

ning of each Remote Read DMA operation the value of

pkt will be loaded into RSAR0 and RSAR1

2 When initializing the NIC set

BNDRY e PSTART

CURR e PSTART a 1

pkt e PSTART a 1

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