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Intel
®
21143 PCI/CardBus 10/100Mb/s Ethernet LAN Controller
20 Design Guide
4.3 Crystal and Crystal Oscillator Connections
Figure 10 shows two serial clock connections; select either the crystal connection or the crystal
oscillator connection. According to the IEEE 802.3 standard, a 20 MHz crystal is required. The
crystal frequency must not vary by more than 100 parts per million (PPM), or 0.01%. Place the
crystal as close as possible to the 21143.
Because the frequency of crystals from different vendors can vary, test the crystals in the actual circuit. It
may be necessary to vary the tuning of the surrounding components. However, after the capacitors have
been tuned for the specific crystal, the design does not need to be altered on a board-by-board basis.
The 21143 also supports a crystal oscillator (Figure 10). This configuration requires no external
component and xtal2 should be left open.This is useful for applications with multiple network connections.
Table 12 lists the crystal specifications.
Figure 10. 21143 External Component Connections
B0033-01
xtal1
xtal2
xtal1
xtal2
Intel
®
21143
Ethernet Controller
Crystal
Connection
Crystal
Oscillator
Connection
Current
Reference
Connection
iref
vcap_h
100
Ω
20-MHz Input Clock
GND
GND
No Connection
10 k
Ω
120 pF
82 pF
2.4 k
Ω
20-MHz
Crystal
0.022 µF
GND
106
105
106
107
108
110
Table 12. Crystal Specifications
Specification Value Units
Crystal frequency 20.000 MHz
Frequency tolerance
±50 PPM
Load capacitance 50 pF
Frequency stability
±30 PPM
Maximum effective series resistance 40 Ohms (
Ω)
Test condition drive level 100 µW