DP8390D datasheet(3/56 Pages) NSC

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

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