NXP Semiconductors freescale semiconductor MC13211, Reference Manual

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Modem Miscellaneous Functions
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor 9-5
Figure 9-2. Crystal Frequency Variation vs. xtal_trim[7:0]
Figure 9-2 shows typical oscillator frequency decrease versus the value programmed in xtal_trim[7:0].
9.4 Output Clock Pin CLKO
The MC1321x can supply a clock output useful as a frequency source for a microcontroller, frequency test
point, or reference for other uses. The clock output is available on signal CLKO and can be turned on or
off as a power saving measure (default is CLKO active). CLKO is controlled by a number of control fields.
9.4.1 Enable CLKO (clko_en, Control_C Register 09, Bit 5)
Setting clko_en, Contro_C Register 9, Bit 5, to 1 enables the CLKO signal. The default condition out of
reset is that the clock out is enabled at the default frequency of 32.768+ kHz set by field clko_rate[2:0].
9.4.2 Setting CLKO frequency (clko_rate[2:0], CLKO_Ctl Register 0A, Bits
2-0)
The 3-bit field clko_rate[2:0], CLKO_Ctl Register 0A, Bits 2-0, selects the output frequency based on the
programmed value. Frequencies from 16 MHz to 16 kHz are available. Default frequency is 32.768+ kHz
with a field value of clko_rate[2:0] = 110.
Table 5-13 lists the CLKO frequencies versus clko_rate[2:0]
program value.
- 900
- 800
- 700
- 600
- 500
- 400
- 300
- 200
- 100
0
0 50 100 150 200 250 300
xtal_trim[7:0] (decimal)
Frequency Decrease (Hz)