MC13211 Datasheet, PDF (22/70 Pages) Freescale Semiconductor, Inc – ZigBee™- Compliant Platform - 2.4 GHz Low Power Transceiver for the IEEE® 802.15.4 Standard plus Microcontroller

4.7 Modem Crystal Oscillator
The modem crystal oscillator uses the following external pins as shown in Figure 12.
1. XTAL1 - reference oscillator input.
2. XTAL2 - reference oscillator output. Note that this pin should not be loaded as a reference source
or to measure frequency; instead use CLKO to measure or supply 16 MHz.
MC1321X
802.15.4 MODEM
XTAL1
XTAL2 CLKO
27
28
10
16MHz
Figure 12. Modem Crystal Oscillator
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 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
Freescale has specified that a 16 MHz crystal with a <9 pF load capacitance is required. The 802.15.4
modem 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
802.15.4 modem 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.
The modem uses the 16 MHz crystal oscillator as the reference oscillator for the system and a
programmable warp capability is provided. It is controlled by programming CLKO_Ctl Register 0A, Bits
MC13211/212/213 Technical Data, Rev. 1.5
22
Freescale Semiconductor