Modem Miscellaneous Functions
MC1321x Reference Manual, Rev. 1.6
9-4
Freescale Semiconductor
Figure 9-1. Crystal Oscillator Circuit
9.3.1
Crystal Requirements
The MC1321x requires that only a 16 MHz crystal with a <9 pF load capacitance can be used. The load
capacitance limitation is required due to internal oscillator circuit and the ability to trim the oscillator as
described in the next section. A tight frequency tolerance on the crystal may also be required due to the
802.15.4 Standard which demands that frequency tolerances be kept /- 40 ppm. This requirement
is for the oscillator circuit, not just the crystal. This is covered in detail in the appropriate MC1321x Data
Sheet.
9.3.2
Crystal Trim Operation
The MC1321x uses the 16 MHz crystal oscillator with warp capability as the reference oscillator for the
system. The warp capability is done by the MC1321x and is controlled by programming CLKO_Ctl
Register 0A, Bits 15-8 (xtal_trim[7:0]). The trimming procedure varies the frequency by a few hertz per
step, depending on the type of crystal. The high end of the frequency spectrum is set when xtal_trim[7:0]
is set to zero. As xtal_trim[7:0] is increased, the frequency is decreased. Accuracy of this feature can be
observed by varying xtal_trim[7:0] and using a spectrum analyzer or frequency counter to track the change
in frequency of the crystal signal. The reference oscillator frequency can be measured at the CLKO contact
by programming CLKO_Ctl Register 0A, Bits 2-0, to value 000. The crystal frequency should not be
monitored at IC pins 26 or 27 (XTAL1 or XTAL2) because this will load the oscillator and alter the
frequency.
XTAL1
XTAL2
MC1321x
C2
C1
CRYSTAL
Содержание freescale semiconductor MC13211
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