LAN8720AI datasheet(74/79 Pages) SMSC

74 / 79 page

Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

Revision 1.4 (08-23-12)

SMSC LAN8720A/LAN8720Ai

DATASHEET

5.6

Clock Circuit

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

input. 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. See Table 5.13 for the

recommended crystal specifications.

Note 5.18 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 5.19 Frequency Deviation Over Time is also referred to as Aging.

Note 5.20 The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3u as

±100 PPM.

Note 5.21 0oC for extended commercial version, -40oC for industrial version.

Note 5.22 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 5.13 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 5.18

Frequency Stability Over Temp

Ftemp

±50

PPM

Note 5.18

Frequency Deviation Over Time

Fage

+/-3 to 5

PPM

Note 5.19

Total Allowable PPM Budget

±50

PPM

Note 5.20

Shunt Capacitance

-7 typ-

Load Capacitance

20 typ

Drive Level

300

Equivalent Series Resistance

Ohm

Operating Temperature Range

Note 5.21

-+85

XTAL1/CLKIN Pin Capacitance

3 typ

Note 5.22

XTAL2 Pin Capacitance

3 typ

Note 5.22

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