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ICS8714004I Datasheet(PDF) 18 Page - Integrated Device Technology |
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ICS8714004I Datasheet(HTML) 18 Page - Integrated Device Technology |
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18 / 31 page  ICS8714004DKI REVISION A MARCH 24, 2014 18 ©2014 Integrated Device Technology, Inc. ICS8714004I Data Sheet FemtoCLock® Zero Delay Buffer/Clock Generator for PCI Express™ and Ethernet Overdriving the XTAL Interface The XTAL_IN input can be overdriven by an LVCMOS driver or by one side of a differential driver through an AC coupling capacitor. The XTAL_OUT pin can be left floating. The amplitude of the input signal should be between 500mV and 1.8V and the slew rate should not be less than 0.2V/nS. For 3.3V LVCMOS inputs, the amplitude must be reduced from full swing to at least half the swing in order to prevent signal interference with the power rail and to reduce internal noise. Figure 3A shows an example of the interface diagram for a high speed 3.3V LVCMOS driver. This configuration requires that the sum of the output impedance of the driver (Ro) and the series resistance (Rs) equals the transmission line impedance. In addition, matched termination at the crystal input will attenuate the signal in half. This can be done in one of two ways. First, R1 and R2 in parallel should equal the transmission line impedance. For most 50 applications, R1 and R2 can be 100 . This can also be accomplished by removing R1 and changing R2 to 50 . The values of the resistors can be increased to reduce the loading for a slower and weaker LVCMOS driver. Figure 3B shows an example of the interface diagram for an LVPECL driver. This is a standard LVPECL termination with one side of the driver feeding the XTAL_IN input. It is recommended that all components in the schematics be placed in the layout. Though some components might not be used, they can be utilized for debugging purposes. The datasheet specifications are characterized and guaranteed by using a quartz crystal as the input. Figure 3A. General Diagram for LVCMOS Driver to XTAL Input Interface Figure 3B. General Diagram for LVPECL Driver to XTAL Input Interface VCC XTAL_OUT XTAL_IN R1 100 R2 100 Zo = 50 ohms Rs Ro Zo = Ro + Rs C1 .1uf LVCMOS Driver XTAL_OUT XTAL_IN Zo = 50 ohms C2 .1uf LVPECL Driver Zo = 50 ohms R1 50 R2 50 R3 50 |
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