PXN20RM Datasheet, PDF (177/1376 Pages) Freescale Semiconductor, Inc – PXN20 Microcontroller

Frequency Modulated Phase-Locked Loop (FMPLL)
2. Write a value of ERFD = ERFD (from step 1) + 1 to the ERFD field of the ESYNCR2. Not
increasing the ERFD when changing the EPREDIV or EMFD could subject the device to clock
frequencies beyond the range specified for the device due to the PLL’s unlocked state.
3. If frequency modulation is currently enabled, disable it by writing 00 to the EDEPTH field of the
ESYNCR2.
4. If programming the EPREDIV and/or EMFD, write the value(s) determined in step 1 to the
appropriate field(s) in the ESYNCR1.
5. Monitor the synthesizer lock bit (LOCK) in the synthesizer status register (SYNSR). When the
PLL achieves lock, write the ERFD value determined in step 1 to the ERFD field of the ESYNCR2.
This changes the system clocks frequency to the desired frequency. If frequency modulation is
desired, leave ERFD programmed to ERFD + 1 until after completing the steps in
Section 7.4.3.4.2, Programming System Clock Frequency With Frequency Modulation.
6. If frequency modulation was enabled initially, it can be re-enabled following the steps listed in
Section 7.4.3.4.2, Programming System Clock Frequency With Frequency Modulation.
7.4.3.4 PLL Normal Mode With Frequency Modulation
In normal PLL clock mode, frequency modulation is not enabled in the default synthesis mode. When
frequency modulation is enabled two parameters must be set to generate the desired level of modulation.
The parameters to be programmed are the RATE and DEPTH bit fields of the ESYNCR2 register. The
RATE bit controls the frequency of modulation, F_mod. The DEPTH bits work to control the modulation
depth, F_m. The available modulation rates and depths are given in Table 7-8 and Table 7-9, respectively.
The modulation waveform is always a triangle wave and its shape is not programmable. An example of
one period of the modulation waveform is shown in Figure 7-6.
Freescale Semiconductor
PXN20 Microcontroller Reference Manual, Rev. 1
7-15