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ZL50019 Datasheet(PDF) 84 Page - Zarlink Semiconductor Inc |
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ZL50019 Datasheet(HTML) 84 Page - Zarlink Semiconductor Inc |
84 / 115 page ZL50019 Data Sheet 84 Zarlink Semiconductor Inc. 26.0 Applications This section contains application-specific details for clock and crystal operation and power supply decoupling. 26.1 OSCi Master Clock Requirement The device requires a 20 MHz master clock source at the OSCi pin when operating in Master mode or in Divided Slave with OSC mode. The clock source may be either an external clock oscillator connected to the OSCi pin, or an external crystal connected between the OSCi and OSCo pins. If an external clock source is present, OSC_EN must be tied high. Note that using a crystal is only suitable for wider tolerance applications (i.e., ±100 ppm). For stratum 4E applications a clock oscillator with a tolerance of ±32 ppm should be used. See Application Note ZLAN-68 for a list of Oscillators and Crystals that can be used with Zarlink PLL’s and Digital Switches with embedded PLL’s. 26.1.1 External Crystal Oscillator When an external crystal oscillator is used, a complete oscillator circuit made up of a crystal, resistor and capacitors is shown in Figure 21 on page 84. XC is a buffered version of the 20 MHz input clock connected to the internal circuitry. Figure 21 - Crystal Oscillator Circuit The accuracy of a crystal oscillator circuit depends on the crystal tolerance as well as the load capacitance tolerance. Typically, for a 20 MHz crystal specified with a 32 pF load capacitance, each 1 pF change in load capacitance contributes approximately 9 ppm to the frequency deviation. Consequently, capacitor tolerances and stray capacitances have a major effect on the accuracy of the oscillator frequency. The trimmer capacitor shown in Figure 21 on page 84 may be used to compensate for capacitive effects. The crystal should be a fundamental mode type - not an overtone. The fundamental mode crystal permits a simpler oscillator circuit with no additional filter components and is less likely to generate spurious responses. The crystal accuracy only affects the output clock accuracy in the freerun or the holdover mode. The crystal specification is as follows: Frequency 20 MHz Tolerance As required Oscillation Mode Fundamental Resonance Mode Parallel Load Capacitance 20 pF - 32 pF OSCo 25 pF 1M Ω 25 pF 20 MHz OSCi XC 2K DX |
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