
System Considerations
MC1321x Reference Manual, Rev. 1.6
3-8
Freescale Semiconductor
A monitor point for the modem clock frequency - the CLKO is a buffered output that can be used
to observe the modem frequency for calibration and design purposes. Modem oscillator pins
XTAL1 and XTAL2 cannot be used for monitoring the oscillator frequency because this will load
the circuit and cause either a frequency error or shutdown the oscillator.
The CLKO output frequency is controlled by programming the modem 3-bit field clko_rate[2:0],
CLKO_Ctl Register 0A, Bits 2-0. Default frequency is 32.786+ kHz with a field value of clko_rate[2:0] =
110.
Ταβλε 512 lists the CLKO frequencies versus clko_rate[2:0] program value. The use and
3.5.3
MCU Clock Sources
The MCU has several options for its primary clock source depending on its mode of operation as well as
its hardware configuration. A detailed discussion of the MCU ICG and associated clock sources is in
3.5.3.1
MCU External Clock Source
As shown in
Figure 3-3, the external pins associated with the MCU clock source are output XTAL and
input EXTAL. An external clock source can have a frequency as high as 40 MHz with no minimum
frequency. The external source must have compatible logic levels and drive input EXTAL. Note that no
external components are required for the clock oscillator if an external source is used.
3.5.3.2
MCU External Crystal Oscillator
Another choice for the MCU oscillator is the use of an external crystal or ceramic resonator as shown in
Figure 3-3. The MCU oscillator is a Pierce type that can accommodate crystals or resonators in either of
two frequency ranges (low range and high range) as shown in
Table 3-3. Note that the frequency range
must be selected properly by programming the RANGE bit in the MCU ICGC1 register (the default
condition is the high range).
External resistor RF provides a bias path for the oscillator to keep the input in its linear range. The typical
value for RF is dependent on the frequency range and is shown in
Table 3-3 although its value is typically
not critical. Resistor RS is typically not used and is only required if the crystal or resonator needs
protection from the power of the oscillator output.
Table 3-3. MCU Oscillator Components
Characteristic
Symbol
Min
Typ1
1 Data in Typical column was characterized at 3.0 V, 25
°C or is typical recommended value.
Max
Unit
Load capacitors
C3
C4
2
2 See crystal or resonator manufacturer’s recommendation.
Feedback resistor
Low range (32 kHz - 100 kHz)
High range (1 MHz – 16 MHz)
RF
10
1
M
Ω
M
Ω
Series Resistor
RS
0
Ω