DP83936AVF datasheet(76/104 Pages) NSC

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80 Network Interfacing (Continued)

TABLE 8-1 Crystal Specifications

Resonant frequency

20 MHz

Tolerance (see text)

001% at 25 C

Accuracy

0005% (50 ppm) at 0 to 70 C

Fundamental Mode Series Resistance

25X

Specified Load Capacitance

18 pF

Type

AT cut

Circuit

Parallel Resonance

8132 Clock Oscillator Module

The SONIC-T also allows an external clock oscillator to be

used The connection configuration is shown in

Figure 8-5

This connection requires an oscillator with the following

specifications

1 TTL or CMOS output with a 001% frequency tolerance

2 40% – 60% duty cycle

3 One CMOS load output drive

These specifications assume that no other circuitry is driv-

en In this configuration the OSCOUT pin must be left open

TLF12597 – 55

FIGURE 8-5 Oscillator Module Connection

to the SONIC-T

8133 PCB Layout Considerations

Care should be taken when connecting a crystal Stray ca-

pacitance (eg from PC board traces and plated through-

holes around the OSCIN and OSCOUT pins) can shift the

crystal’s frequency out of range causing the transmitted fre-

quency to exceed the 001% tolerance specified by IEEE

The layout considerations for using an external crystal are

rather straightforward The oscillator layout should locate all

components close to the OSCIN and OSCOUT pins and

should use short traces that avoid excess capacitance and

inductance A solid ground should be used to connect the

ground legs of the two capacitors

When connecting an external oscillator the only considera-

tions are to keep the oscillator module as close to the

SONIC-T as possible to reduce stray capacitance and in-

ductance and to give the module a clean VCC and a solid

ground

814 Power Supply Considerations

In general

power supply routing and design for the

SONIC-T need only follow standard practices In some situ-

ations however additional care may be necessary in the

layout of the analog supply Specifically special care may be

needed for the TXVCC RXVCC PLLVCC OSCVCC RXTVCC

and TPVCC power supplies and the TXGND RXGND

PLLGND OSCGND TPGND and ANGND In most cases

the analog and digital power supplies can be interconnect-

However

to ensure optimum performance of the

SONIC-T’s analog functions power supply noise should be

minimized To reduce analog supply noise any of several

techniques can be used

1 Route analog supplies as a separate set of traces or

planes from the digital supplies with their own decoupling

capacitors

2 Provide noise filtering on the analog supply pins by insert-

ing a low pass filter Alternatively a ferrite bead could be

used to reduce high frequency power supply noise

3 Utilize a separate regulator to generate the analog sup-

ply

82 TWISTED PAIR INTERFACE MODULE

Transmitter Considerations

The transmitter consists of

four signals the true and complement Manchester encoded

data (TXOg) and these signals delayed by 50 ns (TXOdg)

These four signals are resistively combined

(Figure 8-6)

TXOa with TXOdb and TXOb with TXOda in a configu-

ration referred to as pre-emphasis This digital pre-emphasis

is required to compensate for the low-pass filter effects of

the twisted pair cable which cause greater attenuation to

the 10 MHz (50 ns) pulses of the Manchester encoded

waveform than the 5 MHz (100 ns) pulses

TLF12597 – 56

FIGURE 8-6 External Circuitry to Connect the SONIC-T to Twisted Pair Cable

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