SMSC LAN9312 - Manual (Page 453)

High Performance Two Port 10/100 Managed Ethernet Switch with 32-Bit Non-PCI CPU Interface
Datasheet
SMSC LAN9312 453 Revision 1.4 (08-19-08)
DATASHEET
15.6 Clock Circuit
The LAN9312 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 15.15 for crystal specifications.
Note 15.11 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 15.12 Frequency Deviation Over Time is also referred to as Aging.
Note 15.13 The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3 as
+/- 50 PPM.
Note 15.14 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 15.15 LAN9312Crystal 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
C F
tol
- - +/-50 PPM Note 15.11
Frequency Stability Over Temp F
temp
- - +/-50 PPM Note 15.11
Frequency Deviation Over Time F
age
- +/-3 to 5 - PPM Note 15.12
Total Allowable PPM Budget - - +/-50 PPM Note 15.13
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
- - 30 Ohm
Operating Temperature Range 0 - +70 o C
n/a XI Pin Capacitance - 3 typ - pF Note 15.14
n/a XO Pin Capacitance - 3 typ - pF Note 15.14