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Datasheet
Revision 1.22 (09-25-08) 166 SMSC LAN9420/LAN9420i
DATASHEET
5.9 Clock Circuit
LAN9420/LAN9420i can accept either a 25MHz crystal (preferred) or a 25MHz single-ended clock
oscillator (+/- 50ppm) input. If the single-ended clock oscillator method is implemented, XO should be
left unconnected and XI should 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 5.13 for the recommended crystal specifications.
Note 5.17 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 5.18 Frequency Deviation Over Time is also referred to as Aging.
Note 5.19 The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3u as
+/- 50 PPM.
Note 5.20 0
o
C for commercial version, -40
o
C for industrial version.
Note 5.21 +70
o
C for commercial version, +85
o
C 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 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 5.13 LAN9420/LAN9420i 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 5.17
Frequency Stability Over Temp F
temp
- - +/-50 PPM Note 5.17
Frequency Deviation Over Time F
age
- +/-3 to 5 - PPM Note 5.18
Total Allowable PPM Budget - - +/-50 PPM Note 5.19
Shunt Capacitance C
O
-7 typ-pF
Load Capacitance C
L
- 20 typ - pF
Drive Level P
W
0.5 - - mW
Equivalent Series Resistance R
1
--30Ohm
Operating Temperature Range Note 5.20 - Note 5.21
o
C
LAN9420/LAN9420i XI Pin
Capacitance
-3 typ-pFNote 5.22
LAN9420/LAN9420i XO Pin
Capacitance
-3 typ-pFNote 5.22