Freescale Semiconductor MPC5553 - Page 444

MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 11-29
Figure 11-10. Frequency Modulation Waveform
11.4.3.3 FM Calibration Routine
Upon enabling frequency modulation, a new calibration routine is performed. This routine tunes a
reference current into the modulation D/A so that the modulation depth (F
max
and F
min
) remains within
specification.
Entering the FM calibration mode requires the user to program SYNCR[EXP]. The EXP is the expected
value of the difference between the reference and feedback counters used in the calibration of the FM
equation:
For example, if 80 MHz is the desired final frequency and 8 MHz crystal is used, the final values of
MFD=6 and RFD=0 will produce the desired 80 MHz. For a desired frequency modulation with a 1%
depth, then EXP is calculated using P = 1, MFD = 6 and M = 480. Refer to Table 11-11 for a complete list
of values to be used for the variable (M) based on MFD setting. To obtain a percent modulation (P) of 1%,
the EXP field would have to be set at:
Rounding this value to the closest integer yields the value of 48 that should be entered into the EXP field
for this example.
Table 11-11. Multiplied Factor Dividers with M Values
MFD M
0-2 960
3-5 640
F
max
= F
sys
+ {1%, 2%}
F
min
= F
sys
– {1%, 2%}
F
mod
= F
ref_crystal
or F
ref_ext
/ (PREDIV + 1) Q where Q = 40 or 80
F
max
Fm
t
F
min
f
t =
1
F
mod
Fm
EXP
MFD 4+MP
100
--------------------------------------------------
=
EXP 6 4+480 1100 48==