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Frequency-modulated phase locked loop (FMPLL)
MPC5644A Microcontroller Reference Manual, Rev. 6
580
Freescale Semiconductor
The following equations define how to calculate MODPERIOD and INCSTEP based on the frequency of
the feedback divider (f
fbk
), the modulation frequency (f
mod
) and the modulation depth percentage (MD):
Eqn. 17-5
Eqn. 17-6
MODPERIOD and INCSTEP are subject to the following restriction:
Eqn. 17-7
Because of the above rounding operations, the effective modulation depth applied to the FMPLL is given
by the following formula:
Eqn. 17-8
As an example, suppose the following configuration:
•
Input frequency: 4 MHz
•
Load divider (EMFD): 64
•
Input divider: 1
•
VCO frequency: 4 MHz × 64 = 256 MHz
•
PLL output frequency: 256 MHz / ERFD
•
Center spread (MODSEL = 0)
•
Modulation frequency: 24 kHz
•
Modulation depth: +/
2.0 % (4% peak-to-peak)
•
MODPERIOD = Round [(4 × 10
6
)/(4 × 24 × 10
3
)] = Round [41.66] = 42
•
INCSTEP = Round [((2
15
1) × 2 × 64) / (100 × 5 × 42)] = Round [199.722] = 200
•
MODPERIOD × INCSTEP = 42 × 200 = 8400 (which is less than 2
15
)
•
MD (quantized) = ((42 × 200 × 100 × 5) / ((2
15
1) × 64) = 2.00278 %
In this example, the modulation depth error is 0.00278%.
The FM parameters can only be changed, and FM can only be enabled, when the PLL is locked. Writing
to the FMPLL_SYNFMMR while the PLL is unlocked has no effect. Furthermore, when the PLL loses
MODPERIOD
round
f
fbk
4
f
mod
------------------
(
)
=
INCSTEP
round
2
15
1
–
MD
EMFD
100
5
MODPERIOD
---------------------------------------------------------------
(
)
=
MODPERIOD
INCSTEP
2
15
INCSTEP
round
MODPERIOD
INCSTEP
100
5
2
15
1
–
EMFD
-----------------------------------------------------------------------------------------------
(
)
=
Содержание MPC5644A
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