DP83905 datasheet(23/80 Pages) NSC

23 / 80 page

40 Functional Description (Continued)

Status Information

Status information is provided by the ATLANTIC Controller

on the RXLED TXLED COLED and POL outputs as de-

scribed in the pin description table These outputs are suit-

able for driving status LED’s as shown in

Figure 17 All out-

puts are open drain

Recommended integrated Filter-Transformer-choke mod-

ules

1 Pulse Engineering PE65424

2 Valor FL1012 or FL1030

49 ENCODERDECODER (ENDEC) MODULE

The ENDEC consists of four main logical blocks

a The oscillator generates the 10 MHz transmit clock sig-

nal for system timing

b The Manchester encoder accepts NRZ data from the

controller encodes the data to Manchester and trans-

mits the data differentially to the transceiver through the

differential transmit driver

c The Manchester decoder receives Manchester data from

the transceiver converts it to NRZ data and clock puls-

es and sends it to the controller

d The collision translator indicates to the controller the

presence of a valid 10 MHz collision signal to the PLL

Oscillator

The oscillator is controlled by a 20 MHz parallel resonant

crystal connected between X1 and X2 or by an external

clock on X1 The 20 MHz output of the oscillator is divided

by 2 to generate the 10 MHz transmit clock for the control-

ler The oscillator also provides internal clock signals to the

encoding and decoding circuits

CRYSTAL OPERATION

If the crystal used with the internal oscillator circuit is not

properly selected the ATLANTIC Controller oscillator may

not reliably start oscillation under all conditions

If this occurs it could be deceiving to a designer since his

prototypes may work fine However when the designer

does qualification testing or starts production he may en-

counter a higher than expected board yield loss due to the

oscillator not starting The ATLANTIC Controller’s oscilla-

tor

circuit

clocks

the

Encoder-Decoder

logic

The

ATLANTIC Controller’s oscillator also clocks the twisted

pair interface block If the oscillator does not start the

ATLANTIC Controller will not be able to transmit or re-

ceive

If a crystal is connected to the ATLANTIC Controller it is

recommended that the circuit shown in

Figure 19 be used

and that the components used meet the following

Crystal XT1 AT cut parallel resonant crystal

Series Resistance s25X

Specified Load Capacitance s20 pF

Accuracy 0005% (50 ppm)

Typical Load 50 mW–75 mW

The recommended values for capacitors C1 and C2 are

26 pF minus the board capacitance on that pin Therefore if

both X1 and X2 have 4 pF of board capacitance then a

22 pF capacitor should be used

According to the IEEE 8023 standard the entire oscillator

circuit (crystal and amplifier) must be accurate to 001%

When using a crystal the X2 pin is not guaranteed to pro-

vide a TTL compatible logic output and should not be used

TLF11498 – 16

FIGURE 19 Crystal Connection to ATLANTIC

Controller (see text for component values)

to drive external standard logic If additional logic needs to

be driven then an external oscillator should be used as

described in the following section

OSCILLATOR MODULE OPERATION

If the designer wishes to use a crystal clock oscillator one

that provides the following should be employed

1 TTL or CMOS output with a 001% frequency tolerance

2 40% – 60% duty cycle

The circuit is shown in

Figure 20 When using a clock oscil-

lator it is recommended that the designer connect the oscil-

lator output to the X1 pin and leave the X2 pin floating

TLF11498 – 17

FIGURE 20 ATLANTIC Controller

Connection for Oscillator Module

Manchester Encoder and Differential Driver

The differential transmit pair on the secondary of the em-

ployed transformer drives up to 50 meters of twisted pair

AUI cable These outputs are source followers which require

two 270X pull-down resistors to ground (see

Figure 21 )

The ATLANTIC Controller allows full-step to be compatible

with IEEE 8023 Transmita and Transmitb are equal in the

idle state providing zero differential voltage to operate with

transformer coupled loads

Manchester Decoder

The decoder consists of a differential receiver and a PLL to

separate a Manchester encoded data stream into internal

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