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MC145151-2 View Datasheet(PDF) - Freescale Semiconductor

Part Name
Description
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MC145151-2
Freescale
Freescale Semiconductor Freescale
MC145151-2 Datasheet PDF : 24 Pages
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Design Considerations
4.2 Crystal Oscillator Considerations
The following options may be considered to provide a reference frequency to Freescale's CMOS frequency
synthesizers.
4.2.1 Use of a Hybrid Crystal Oscillator
Commercially available temperature-compensated crystal oscillators (TCXOs) or crystal-controlled data
clock oscillators provide very stable reference frequencies. An oscillator capable of sinking and sourcing
50 ยตA at CMOS logic levels may be direct or dc coupled to OSCin. In general, the highest frequency
capability is obtained utilizing a direct-coupled square wave having a rail-to-rail (VDD to VSS) voltage
swing. If the oscillator does not have CMOS logic levels on the outputs, capacitive or ac coupling to OSCin
may be used. OSCout, an unbuffered output, should be left floating.
4.2.2 Design an Off-Chip Reference
The user may design an off-chip crystal oscillator using ICs specifically developed for crystal oscillator
applications, or using discrete transistors. The reference signal from the oscillator is ac coupled to OSCin.
For large amplitude signals (standard CMOS logic levels), dc coupling is used. OSCout, an unbuffered
output, should be left floating. In general, the highest frequency capability is obtained with a
direct-coupled square wave having rail-to-rail voltage swing.
4.2.3 Use of the On-Chip Oscillator Circuitry
The on-chip amplifier (a digital inverter) along with an appropriate crystal may be used to provide a
reference source frequency. A fundamental mode crystal, parallel resonant at the desired operating
frequency, should be connected as shown in Figure 13.
Rf
FREQUENCY
SYNTHESIZER
OSCin
R1* OSCout
C1
C2
* May be deleted in certain cases. See text.
Figure 13. Pierce Crystal Oscillator Circuit
MC145151-2 and MC145152-2 Technical Data, Rev. 5
Freescale Semiconductor
19
 

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