LAN8804 datasheet(80/91 Pages) MICROCHIP

80 / 91 page

LAN8804

DS00003591G-page 80

6.7

Clock Circuit

The device can accept either a 25 MHz crystal (preferred) or a 25 MHz single-ended clock oscillator (± 50 ppm, max

jitter 100 ps rms) input. If the single-ended clock oscillator method is implemented,

XO must be left unconnected and

XI must be driven with a nominal 0-3.3V clock signal. The input clock duty cycle is 40% minimum, 50% typical and 60%

maximum.

It is recommended that a crystal utilizing matching parallel load capacitors be used for the crystal input/output signals

(

XI/XO). See Table 6-22 for the recommended crystal specifications.

Note 6-23

The maximum allowable values for Frequency Tolerance and Frequency Stability are application

dependent. Since any particular application must meet the IEEE ±50 ppm Total PPM Budget, the

combination of these two values must be approximately ±45 ppm (allowing for aging).

Note 6-24

Frequency Deviation Over Time is also referred to as Aging.

Note 6-25

The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3u as ±50 ppm.

Note 6-26

0oC ambient for commercial version, -40oC ambient for industrial version.

Note 6-27

+85oC ambient.

Note 6-28

This number includes the pad, the bond wire and the lead frame. PCB capacitance is not included

in this value. The

XO/XI pin and PCB capacitance values are required to accurately calculate the

value of the two external load capacitors. These two external load capacitors determine the accuracy

of the 25.000 MHz frequency.

TABLE 6-22: CRYSTAL SPECIFICATIONS

Parameter

Symbol

Min

Nom

Max

Units

Notes

Crystal Cut

AT, typ

—

Crystal Oscillation Mode

Fundamental Mode

Crystal Calibration Mode

Parallel Resonant Mode

Frequency

Ffund

—

25.000

—

MHz

Frequency Tolerance @ 25oC

Ftol

—

±50

ppm

Note 6-23

Frequency Stability Over Temp

Ftemp

—

±50

ppm

Note 6-23

Frequency Deviation Over Time

Fage

—

±3 to 5

—

ppm

Note 6-24

Total Allowable PPM Budget

—

±50

ppm

Note 6-25

Shunt Capacitance

—

Load Capacitance

—

Motional Inductance

—

Drive Level

—

100

µW

Equivalent Series Resistance

—

Ohm

Operating Temperature Range

—

Note 6-26

—

Note 6-27

XI Pin Capacitance

—

-—

2 typ

—

Note 6-28

XO Pin Capacitance

—

2 typ

—

Note 6-28

Crystal Startup Time

tXSO

—

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