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DP8390 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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DP8390 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 56 page  70 Packet Reception (Continued) 3 After a packet is DMAed from the Receive Buffer Ring the Next Page Pointer (second byte in NIC buffer header) is used to update BNDRY and next pkt next pkt e Next Page Pointer BNDRY e Next Page Pointer b 1 If BNDRY k PSTART then BNDRY e PSTOP b 1 Note the size of the Receive Buffer Ring is reduced by one 256-byte buffer this will not however impede the operation of the NIC In StarLAN applications using bus clock frequencies greater than 4 MHz the NIC does not update the buffer header information properly because of the disparity between the network and bus clock speeds The lower byte count is cop- ied twice into the third and fourth locations of the buffer header and the upper byte count is not written The upper byte count however can be calculated from the current next page pointer (second byte in the buffer header) and the previous next page pointer (stored in memory by the CPU) The following routine calculates the upper byte count and allows StarLAN applications to be insensitive to bus clock speeds Next pkt is defined similarly as above 1st Received Packet Removed By Remote DMA TLF8582 – 57 upper byte count e next page pointer b next pkt b 1 if (upper byte count) k 0 then upper byte count e (PSTOP b next pkt) a (next page pointer b PSTART) b 1 if (lower byte count) l 0 fch then upper byte count e upper byte count a 1 STORAGE FORMAT FOR RECEIVED PACKETS The following diagrams describe the format for how re- ceived packets are placed into memory by the local DMA channel These modes are selected in the Data Configura- tion Register Storage Format AD15 AD8 AD7 AD0 Next Packet Receive Pointer Status Receive Receive Byte Count 1 Byte Count 0 Byte 2 Byte 1 BOS e 0 WTS e 1 in Data Configuration Register This format used with Series 32000 808X type processors AD15 AD8 AD7 AD0 Next Packet Receive Pointer Status Receive Receive Byte Count 0 Byte Count 1 Byte 1 Byte 2 BOS e 1 WTS e 1 in Data Configuration Register This format used with 68000 type processors Note The Receive Byte Count ordering remains the same for BOSe0or1 AD7 AD0 Receive Status Next Packet Pointer Receive Byte Count 0 Receive Byte Count 1 Byte 0 Byte 1 BOS e 0 WTS e 0 in Data Configuration Register This format used with general 8-bit CPUs 80 Packet Transmission The Local DMA is also used during transmission of a pack- et Three registers control the DMA transfer during trans- mission a Transmit Page Start Address Register (TPSR) and the Transmit Byte Count Registers (TBCR01) When the NIC receives a command to transmit the packet pointed to by these registers buffer memory data will be moved into the FIFO as required during transmission The NIC will gen- erate and append the preamble synch and CRC fields TRANSMIT PACKET ASSEMBLY The NIC requires a contiguous assembled packet with the format shown The transmit byte count includes the Destina- tion Address Source Address Length Field and Data It does not include preamble and CRC When transmitting data smaller than 46 bytes the packet must be padded to a minimum size of 64 bytes The programmer is responsible for adding and stripping pad bytes General Transmit Packet Format TLF8582 – 58 11 |
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