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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor
11-27
NOTE
This first register write will cause the FMPLL to switch to an initial system
frequency which is less than the final one. Keeping the change of frequency
to a lower initial value helps minimize the current surge to the external
power supply caused by change of frequency. The last step will be to only
change the RFD to get to the desired final frequency.
NOTE
Changing the MFD or PREDIV values causes the FMPLL to perform a
search for the lock frequency that results in the system clock frequency
changing rapidly across the complete frequency range. All MCU
peripherals, including the external bus will be subjected to this frequency
sweep. Operation of timers and serial communications during this search
sequence will produce unpredictable results.
11.4.3.2
Programming System Clock Frequency with Frequency Modulation
In crystal reference and external reference
clock modes, the default mode is without frequency modulation
enabled. When frequency modulation is enabled, however, three parameters must be set to generate the
desired level of modulation: the RATE, DEPTH, and EXP bit fields of the FMPLL_SYNCR. RATE and
DEPTH determine the modulation rate and the modulation depth. The EXP field controls the FM
calibration routine.
Section 11.4.3.3, “FM Calibration Routine
,” shows how to obtain the values to be
illustrates the effects of the parameters and the modulation waveform
built into the modulation hardware. The modulation waveform is always a triangle wave and its shape is
not programmable.
Note, the modulation rates given are specific to a reference frequency of 8 MHz. F
prediv
is the frequency
after the predivider.
F
mod
= F
ref_crystal
or F
ref_ext
/
( 1) x Q, where Q = 40 or 80. This gives modulation rates of 200
kHz and 100 kHz.
NOTE
The following relationship between F
mod
and modulation rates must be
maintained:
Therefore, the utilization of a non 8 MHz reference will result in scaled modulation rates.
Here are the steps to program the clock frequency with frequency modulation. These steps ensure proper
operation of the calibration routine and prevent frequency overshoot from the sequence:
1. Change the following in FMPLL_SYNCR:
a) Make sure frequency modulation is disabled (FMPLL_SYNCR[DEPTH] = 00). A change to
PREDIV, MFD, or RATE while modulation is enabled will invalidate the previous calibration
results.
b) Clear FMPLL_SYNCR[LOLRE]. If this bit is set, the MCU will go into reset when MFD is
written.
c) Initialize the FMPLL for less than the desired final frequency:
— Disable LOLIRQ.
100 KHz
F
mod
250 KHz
Содержание MPC5553
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