DP83916 datasheet(71/96 Pages) NSC

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60 Network Interfacing (Continued)

TLF11722 – 55

FIGURE 64 Crystal Connection

to the SONIC-16

(see text)

Note 1

The X1 pin is not guaranteed to provide a TTL compatible logic

output and should not be used to drive any external logic If addi-

tional logic needs to be driven then an external oscillator should be

used as described in the following section

Note 2

The frequency marked on the crystal is usually measured with a

fixed load capacitance specified in the crystal’s data sheet The

actual load capacitance used should be the specified value minus

the stray capacitance

TABLE 6-1 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

6132 Clock Oscillator Module

If an external clock oscillator is used the SONIC-16 can be

connected to the external oscillator in one of two ways The

first configuration is shown in

Figure 6-5 In this case an

oscillator that provides the following should be used

1 TTL or CMOS output with a 001% frequency tolerance

2 40% – 60% duty cycle

3 t5 TTL loads output drive (IOL e 8 mA) (Additional out-

put drive may be necessary if the oscillator must also

drive other components)

Again the above assumes no other circuitry is driven

TLF11722 – 56

FIGURE 65 Oscillator Module

Connection to the SONIC-16

The second configuration shown in

Figure 6-6 connects to

the X2 input This connection requires an oscillator with the

same specifications as the previous circuit except that the

output drive specification need only be one CMOS load

This circuit configuration also offers the advantage of slight-

ly lower power consumption In this configuration the X1 pin

must be left open and should not drive external circuitry

Also as shown by

Figure 6-6 there is a 180 phase differ-

ence between connecting an oscillator to X1 compared to

X2 This difference only affects the relationship between

TXC and the oscillator module output The operation of the

SONIC-16 is not affected by this phase change

TLF11722 – 57

FIGURE 66 Alternate Oscillator

Module Connection to the SONIC-16

6133 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 X1 and X2 pins) can shift the crystal’s

frequency out of range causing the transmitted frequency

to exceed the 001% tolerance specified by IEEE The lay-

out considerations for using an external crystal are rather

straightforward The oscillator layout should locate all com-

ponents close to the X1 and X2 pins and should use short

traces that avoid excess capacitance and inductance A sol-

id 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-16 as possible to reduce stray capacitance and in-

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

ground

614 Power Supply Considerations

In general power supply routing and design for the SONIC-

16 need only follow standard practices In some situations

however additional care may be necessary in the layout of

the analog supply Specifically special care may be needed

for the TXVCC RXVCC and PLLVCC power supplies and

the TXGND and ANGND In most cases the analog and

digital power supplies can be interconnected However to

ensure optimum performance of the SONIC-16’s analog

functions power supply noise should be minimized To re-

duce 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

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