Datasheet

LAN8740A/LAN8740Ai
DS00001987A-page 124  2013-2015 Microchip Technology Inc.
5.7 Clock Circuit
The device can accept either a 25 MHz crystal or a 25 MHz 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 0-3.3 V 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
(XTAL1/XTAL2). Either a 300 µW or 100 µW 25 MHz crystal may be utilized. The 300 µW 25 MHz crystal specifications
are detailed in Section 5.7.1, "300 µW 25 MHz Crystal Specifications". The 100 µW 25 MHz crystal specifications are
detailed in Section 5.7.2, "100 µW 25 MHz Crystal Specifications".
5.7.1 300 µW 25 MHZ CRYSTAL SPECIFICATIONS
When utilizing a 300 µW 25 MHz crystal, the following circuit design (Figure 5-8) and specifications (Table 5-15) are
required to ensure proper operation.
Note 1: The maximum allowable values for frequency tolerance and frequency stability are application dependent.
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).
2: Frequency Deviation Over Time is also referred to as Aging.
FIGURE 5-8: 300 µW 25 MHZ CRYSTAL CIRCUIT
TABLE 5-15: 300 µW CRYSTAL SPECIFICATIONS
Parameter Symbol Min. Nom. Max. Unit Note
Crystal Cut AT, typ
Crystal Oscillation Mode Fundamental Mode
Crystal Calibration Mode Parallel Resonant Mode
Frequency F
fund
- 25.000 - MHz
Frequency Tolerance at 25
o
CF
tol
- - ±50 ppm (see Note 1)
Frequency Stability Over Temp F
temp
- - ±50 ppm (see Note 1)
Frequency Deviation Over Time F
age
- ±3 to 5 - ppm (see Note 2)
Total Allowable PPM Budget - - ±50 ppm (see Note 3)
Shunt Capacitance C
O
-7 typ-pF
Load Capacitance C
L
- 20 typ - pF
Drive Level P
W
300 - - µW
Equivalent Series Resistance R
1
--50Ω
Operating Temperature Range (see Note 4)- (see Note 5)
o
C
XTAL1/CLKIN Pin Capacitance - 3 typ - pF (see Note 6)
XTAL2 Pin Capacitance - 3 typ - pF (see Note 6)
LAN8740
XTAL2
XTAL1
Y1
C
1
C
2