DP83907 datasheet(21/70 Pages) NSC

21 / 70 page

40 Functional Description (Continued)

If a crystal is connected to the DP83907 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 s 25X

Specified Load Capacitance s 20 pF

Accuracy 0005% (50 ppm)

Typical Load 50 mW–75 mW

The recommended values for capacitors C1 and C2 should

be 26 pF minus the board capacitance on that pin There-

fore 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

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

TLF12082 – 15

FIGURE 20 DP83907 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

The DP83907 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

TLF12082 – 16

FIGURE 21 Connection from DP83907’s AUI Port to a AUI Connector

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