DP83902A datasheet(12/70 Pages) NSC

12 / 70 page

40 Functional Description (Continued)

3 ST-NIC flushes remaining bytes from FIFO

4 ST-NIC performs internal processing to prepare for writ-

ing the header

5 ST-NIC writes 4-byte (2-word) header

6 ST-NIC de-asserts BREQ

FIFO Threshold Detection

To assure that no overwriting of data in the FIFO the FIFO

logic flags a FIFO overrun as the 13th byte is written into the

FIFO effectively shortening the FIFO to 13 bytes The FIFO

logic also operates differently in Byte Mode and in Word

Mode In Byte Mode a threshold is indicated when the na1

byte has entered the FIFO thus with an 8-byte threshold

the ST-NIC issues Bus Request (BREQ) when the 9th byte

has entered the FIFO For Word Mode BREQ is not gener-

ated until na2 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

Beginning of Transmit

Before transmitting the ST-NIC performs a prefetch from

memory to load the FIFO The number of bytes prefetched

is the programmed FIFO threshold The next BREQ is not

issued until after the ST-NIC actually begins transmitting

data ie after SFD

Reading the FIFO

During normal operation the FIFO must not be read The

ST-NIC will not issue an ACKnowledge back to the CPU if

the FIFO is read The FIFO should only be read during loop-

back diagnostics

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

50 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

ENDEC module and transferred serially to the NIC module

using NRZ data with a clock All fields are of fixed length

except for the data field The ST-NIC generates and ap-

pends the preamble SFD and FCS field during transmis-

sion The Preamble and SFD fields are stripped during re-

ception (The CRC is passed through to buffer memory dur-

ing reception)

PREAMBLE AND START OF FRAME DELIMITER (SFD)

The Manchester encoded alternating 10 preamble field is

used by the ENDEC 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 network The

preamble field is stripped by the NIC module Byte align-

ment is performed with the Start of Frame Delimiter (SFD)

pattern which consists of two consecutive 1’s The ST-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 ST-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 ST-NIC to

accept the packet Multicast addresses begin with an MSB

of ‘‘1’’ The ST-NIC 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

TLF11157 – 7

FIGURE 2

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