Frequency Modulated Phase Locked Loop and System Clocks (FMPLL)
MPC5565 Microcontroller Reference Manual, Rev. 1.0
Freescale Semiconductor
11-27
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.
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, respectively.
NOTE
If a 40 MHz input is used, PLLCFG2 must equal 1. After exiting reset,
PREDIV and any subsequent writes must be
≥
1.
NOTE
The following relationship between F
mod
and modulation rates must be
maintained:
Therefore, the use of a non 8 MHz reference results 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 invalidates the previous calibration
results.
b) Clear FMPLL_SYNCR[LOLRE]. If this bit is set, the MCU goes into reset when MFD is
written.
c) Initialize the FMPLL for less than the desired final frequency:
— Disable LOLIRQ.
— Write FMPLL_SYNCR[PREDIV] to the desired final value.
— Write FMPLL_SYNCR[MFD] to the desired final value.
— Write FMPLL_SYNCR[EXP] to the desired final value.
— Write FMPLL_SYNCR[RATE] to the desired final value.
100 KHz
≤
F
mod
≤
250 KHz
Содержание MPC5565
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