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MC13212 数据表(PDF) 63 Page - Freescale Semiconductor, Inc |
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MC13212 数据表(HTML) 63 Page - Freescale Semiconductor, Inc |
63 / 70 page MC13211/212/213 Technical Data, Rev. 1.5 Freescale Semiconductor 63 7 Application Considerations The following sections describe crystal requirements and RF port options for end user applications. 7.1 Crystal Oscillator Reference Frequency The 802.15.4 Standard requires that several frequency tolerances be kept within ± 40 ppm accuracy. This means that a total offset up to 80 ppm between transmitter and receiver will still result in acceptable performance. The MC1321x transceiver provides onboard crystal trim capacitors to assist in meeting this performance. The primary determining factor in meeting the 802.15.4 Standard, is the tolerance of the crystal oscillator reference frequency. A number of factors can contribute to this tolerance and a crystal specification will quantify each of them: 1. The initial (or make) tolerance of the crystal resonant frequency itself. 2. The variation of the crystal resonant frequency with temperature. 3. The variation of the crystal resonant frequency with time, also commonly known as aging. 4. The variation of the crystal resonant frequency with load capacitance, also commonly known as pulling. This is affected by: a) The external load capacitor values - initial tolerance and variation with temperature. b) The internal trim capacitor values - initial tolerance and variation with temperature. c) Stray capacitance on the crystal pin nodes - including stray on-chip capacitance, stray package capacitance and stray board capacitance; and its initial tolerance and variation with temperature. 5. Whether or not a frequency trim step will be performed in production 7.1.1 Crystal Oscillator Design Considerations Freescale requires that a 16 MHz crystal with a <9 pF load capacitance is used. The MC1321x does not contain a reference divider, so 16 MHz is the only frequency that can be used. A crystal requiring higher load capacitance is prohibited because a higher load on the amplifier circuit may compromise its performance. The crystal manufacturer defines the load capacitance as that total external capacitance seen across the two terminals of the crystal. The oscillator amplifier configuration used in the MC1321x requires two balanced load capacitors from each terminal of the crystal to ground. As such, the capacitors are seen to be in series by the crystal, so each must be <18 pF for proper loading. In the Figure 34 crystal reference schematic, the external load capacitors are shown as 6.8 pF each, used in conjunction with a crystal that requires an 8 pF load capacitance. The default internal trim capacitor value (2.4 pF) and stray capacitance total value (6.8 pF) sum up to 9.2 pF giving a total of 16 pF. The value for the stray capacitance was determined empirically assuming the default internal trim capacitor value and for a specific board layout. A different board layout may require a different external load capacitor value. The on-chip trim capability may be used to determine the closest standard value by adjusting the trim value via the SPI and observing the frequency at CLKO. Each internal trim load capacitor has a trim range of approximately 5 pF in 20 fF steps. Initial tolerance for the internal trim capacitance is approximately ±15%. |
类似零件编号 - MC13212 |
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类似说明 - MC13212 |
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