MPC5604B/C Microcontroller Reference Manual, Rev. 8
Freescale Semiconductor 109
6.7.6.3 Normal mode with frequency modulation
The FMPLL default mode is without frequency modulation enabled. When frequency modulation is
enabled, however, two parameters must be set to generate the desired level of modulation: the PERIOD,
and the STEP. The modulation waveform is always a triangle wave and its shape is not programmable.
FM mode is activated in two steps:
1. Configure the FM mode characteristics: MOD_PERIOD, INC_STEP.
2. Enable the FM mode by programming bit FM_EN of the MR to ‘1’. FM mode can only be enabled
when FMPLL is in lock state.
There are two ways to latch these values inside the FMPLL, depending on the value of bit STRB_BYPASS
in the MR.
If STRB_BYPASS is low, the modulation parameters are latched in the FMPLL only when the strobe
signal goes high for at least two cycles of CLKIN clock. The strobe signal is automatically generated in
the FMPLL digital interface when the modulation is enabled (FM_EN goes high) if the FMPLL is locked
(S_LOCK = 1) or when the modulation has been enabled (FM_EN = 1) and FMPLL enters lock state
(S_LOCK goes high).
If STRB_BYPASS is high, the strobe signal is bypassed. In this case, control bits (MOD_PERIOD[12:0],
INC_STEP[14:0], SPREAD_CONTROL) need to be static or hardwired to constant values. The control
bits must be changed only when the FMPLL is in powerdown mode.
The modulation depth in % is
NOTE
The user must ensure that the product of INCTEP and MODPERIOD is less
than (2
15
-1).
ModulationDepth
100 5 INCSTEPxMODPERIOD
2
15
1–MDF
---------------------------------------------------------------------------------------------
=
To Next Page
