Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
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.
Table 5.13 Crystal Specifications
PARAMETER
SYMBOL
MIN
NOM
AT, typ
Fundamental Mode
Parallel Resonant Mode
MAX
UNITS
NOTES
Crystal Cut
Crystal Oscillation Mode
Crystal Calibration Mode
Frequency
Ffund
Ftol
-
25.000
-
MHz
PPM
PPM
PPM
PPM
pF
Frequency Tolerance @ 25oC
Frequency Stability Over Temp
Frequency Deviation Over Time
Total Allowable PPM Budget
Shunt Capacitance
-
-
±50
Note 5.18
Note 5.18
Note 5.19
Note 5.20
Ftemp
Fage
-
-
±50
-
+/-3 to 5
-
-
-
7 typ
20 typ
-
±50
CO
CL
-
-
Load Capacitance
-
-
-
pF
Drive Level
PW
R1
300
uW
Equivalent Series Resistance
Operating Temperature Range
XTAL1/CLKIN Pin Capacitance
XTAL2 Pin Capacitance
-
-
30
+85
-
Ohm
oC
Note 5.21
-
-
-
3 typ
3 typ
pF
Note 5.22
Note 5.22
-
pF
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.
Revision 1.4 (08-23-12)
74
SMSC LAN8720A/LAN8720Ai
DATASHEET
SMSC [ SMSC CORPORATION ]
