LAN8700I datasheet(57/74 Pages) MICROCHIP

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DS00002260A-page 57

LAN8700/LAN8700i

6.6

Clock Circuit

LAN8700/LAN8700i can accept either a 25MHz crystal or a 25MHz single-ended clock oscillator (±50ppm) input for

operation in MII mode. If the single-ended clock oscillator method is implemented, XTAL2 should be left unconnected

and XTAL1/CLKIN should be driven with a nominal 0-3.3V clock signal. The user is required to supply a 50MHz single-

ended clock for RMII operation. The input clock duty cycle is 40% minimum, 50% typical and 60% maximum. See

Table 6-12 for the recommended crystal specifications.

Note 6-1

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

dependant. 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 6-2

Frequency Deviation Over Time is also referred to as Aging.

Note 6-3

The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3u as

±100 PPM.

Note 6-4

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

Note 6-5

+70oC for commercial version, +85oC for industrial version.

Note 6-6

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

in this value. The XTAL1/CLKIN pin, XTAL2 pin and PCB capacitance values are required to

accurately calculate the value of the two external load capacitors. The total load capacitance must

be equivalent to what the crystal expects to see in the circuit so that the crystal oscillator will operate

at 25.000 MHz.

TABLE 6-12:

LAN8700/LAN8700I 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 @ 25oCFtol

±50

PPM

Note 6-1

Frequency Stability Over Temp

Ftemp

±50

PPM

Note 6-1

Frequency Deviation Over Time

Fage

+/-3 to 5

PPM

Note 6-2

Total Allowable PPM Budget

±50

PPM

Note 6-3

Shunt Capacitance

-7 typ-

Load Capacitance

-20 typ-

Drive Level

0.5

Equivalent Series Resistance

Ohm

Operating Temperature Range

Note 6-4

Note 6-5

Note 6-6

LAN8700/LAN8700i

XTAL1/CLKIN Pin Capacitance

-3 typ-

Note 6-6

LAN8700/LAN8700i XTAL2 Pin

Capacitance

-3 typ-

Note 6-6

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