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DP8390DV Datasheet(PDF) 3 Page - National Semiconductor (TI) |
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DP8390DV Datasheet(HTML) 3 Page - National Semiconductor (TI) |
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3 / 56 page  30 Functional Description (Continued) Because the NIC must buffer the Address field of each in- coming packet to determine whether the packet matches its Physical Address Registers or maps to one of its Multicast Registers the first local DMA transfer does not occur until 8 bytes have accumulated in the FIFO To assure that there is no overwriting of data in the FIFO the FIFO logic flags a FIFO overrun as the 13th byte is written into the FIFO this effectively shortens the FIFO to 13 bytes In addition the FIFO logic operates differently in Byte Mode than in Word Mode In Byte Mode a threshold is indicated when the n a 1 byte has entered the FIFO thus with an 8-byte threshold the NIC issues Bus Request (BREQ) when the 9th byte has entered the FIFO For Word Mode BREQ is not generated until the n a 2 bytes have entered the FIFO Thus with a 4 word threshold (equivalent to an 8-byte threshold) BREQ is issued when the 10th byte has entered the FIFO PROTOCOL PLA The protocol PLA is responsible for implementing the IEEE 8023 protocol including collision recovery with random backoff The Protocol PLA also formats packets during transmission and strips preamble and synch during recep- tion DMA AND BUFFER CONTROL LOGIC The DMA and Buffer Control Logic is used to control two 16-bit DMA channels During reception the Local DMA stores packets in a receive buffer ring located in buffer memory During transmission the Local DMA uses pro- grammed pointer and length registers to transfer a packet from local buffer memory to the FIFO A second DMA chan- nel is used as a slave DMA to transfer data between the local buffer memory and the host system The Local DMA and Remote DMA are internally arbitrated with the Local DMA channel having highest priority Both DMA channels use a common external bus clock to generate all required bus timing External arbitration is performed with a standard bus request bus acknowledge handshake protocol 40 TransmitReceive Packet EncapsulationDecapsulation A standard IEEE 8023 packet consists of the following fields preamble Start of Frame Delimiter (SFD) destination address source address length data and Frame Check Sequence (FCS) The typical format is shown in Figure 2 The packets are Manchester encoded and decoded by the DP8391 SNI and transferred serially to the NIC using NRZ data with a clock All fields are of fixed length except for the data field The NIC generates and appends the preamble SFD and FCS field during transmission The Preamble and SFD fields are stripped during reception (The CRC is passed through to buffer memory during reception) PREAMBLE AND START OF FRAME DELIMITER (SFD) The Manchester encoded alternating 10 preamble field is used by the SNI (DP8391) to acquire bit synchronization with an incoming packet When transmitted each packet contains 62 bits of alternating 10 preamble Some of this preamble will be lost as the packet travels through the net- work The preamble field is stripped by the NIC Byte align- ment is performed with the Start of Frame Delimiter (SFD) pattern which consists of two consecutive 1’s The NIC does not treat the SFD pattern as a byte it detects only the two bit pattern This allows any preceding preamble within the SFD to be used for phase locking DESTINATION ADDRESS The destination address indicates the destination of the packet on the network and is used to filter unwanted pack- ets from reaching a node There are three types of address formats supported by the NIC physical multicast and broadcast The physical address is a unique address that corresponds only to a single node All physical addresses have an MSB of ‘‘0’’ These addresses are compared to the internally stored physical address registers Each bit in the destination address must match in order for the NIC to ac- cept the packet Multicast addresses begin with an MSB of ‘‘1’’ The DP8390D filters multicast addresses using a stan- dard hashing algorithm that maps all multicast addresses into a 6-bit value This 6-bit value indexes a 64-bit array that filters the value If the address consists of all 1’s it is a broadcast address indicating that the packet is intended for all nodes A promiscuous mode allows reception of all pack- ets the destination address is not required to match any filters Physical broadcast multicast and promiscuous ad- dress modes can be selected SOURCE ADDRESS The source address is the physical address of the node that sent the packet Source addresses cannot be multicast or broadcast addresses This field is simply passed to buffer memory LENGTH FIELD The 2-byte length field indicates the number of bytes that are contained in the data field of the packet This field is not interpreted by the NIC DATA FIELD The data field consists of anywhere from 46 to 1500 bytes Messages longer than 1500 bytes need to be broken into multiple packets Messages shorter than 46 bytes will re- quire appending a pad to bring the data field to the minimum length of 46 bytes If the data field is padded the number of valid data bytes is indicated in the length field The NIC does not strip or append pad bytes for short packets or check for oversize packets FCS FIELD The Frame Check Sequence (FCS) is a 32-bit CRC field calculated and appended to a packet during transmission to allow detection of errors when a packet is received During reception error free packets result in a specific pattern in the CRC generator Packets with improper CRC will be re- jected The AUTODIN II (X32 a X26 a X23 a X22 a X16 a X12 a X11 a X10 a X8 a X7 a X5 a X4 a X2 a X1 a 1) polynomial is used for the CRC calculations TLF8582 – 3 FIGURE 2 3 |
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