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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
SMSC LAN8720/LAN8720i 65 Revision 1.0 (05-28-09)
DATASHEET
6.6 Clock Circuit
LAN8720/LAN8720i 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 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 0
o
C for extended commercial version, -40
o
C for industrial version.
Note 6.5 +85
o
C for extended commercial version, +85
o
C 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 LAN8720/LAN8720i 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 F
fund
- 25.000 - MHz
Frequency Tolerance @ 25
o
CF
tol
- - ±50 PPM Note 6.1
Frequency Stability Over Temp F
temp
- - ±50 PPM Note 6.1
Frequency Deviation Over Time F
age
- +/-3 to 5 - PPM Note 6.2
Total Allowable PPM Budget - - ±50 PPM Note 6.3
Shunt Capacitance C
O
-7 typ-pF
Load Capacitance C
L
- 20 typ - pF
Drive Level P
W
300 - - uW
Equivalent Series Resistance R
1
--30Ohm
Operating Temperature Range Note 6.4 - Note 6.5
o
C
LAN8720/LAN8720i
XTAL1/CLKIN Pin Capacitance
-3 typ-pFNote 6.6
LAN8720/LAN8720i XTAL2 Pin
Capacitance
-3 typ-pFNote 6.6