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DP8390DV Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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DP8390DV Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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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 next pkt will be loaded into RSAR0 and RSAR1 2 When initializing the NIC set BNDRY e PSTART CURR e PSTART a 1 next pkt e PSTART a 1 10 |
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Similar Description - DP8390DV |
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