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Frequency-modulated phase locked loop (FMPLL)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
575
I
n legacy mode, the relationship between the VCO frequency
and the output frequency
is determined by the value of
the RFD value programmed in the SYNCR register, according to the following equation:
Eqn. 17-2
In enhanced mode, the relationship between input and output frequency is determined by the EPREDIV, EMFD and ERFD values
programmed in the FMPLL_ESYNCR1 and FMPLL_ESYNCR2, according to the following equation:
Eqn. 17-3
When programming the FMPLL, be sure not to violate the maximum system clock frequency or max/min VCO frequency
specification. In enhanced mode, the VCO frequency is calculated according to the following equation:
Eqn. 17-4
NOTE:
Maximum system clock frequency is 150 MHz and max/min VCO frequency is 256 MHz to 512 MHz.
Furthermore, the PREDIV or EPREDIV values must not be set to any value that causes the input frequency to the phase detector
to go below 4 MHz.
The LOCK flag is immediately negated after any of the following events:
1.
In legacy mode, the PREDIV or MFD fields of the FMPLL_SYNCR are changed
2.
In enhanced mode, the EMODE, EPREDIV, EMFD of CLKCFG[1:2] fields of the FMPLL_ESYNCR1 are changed
1
Upon any of these events an internal timer is initialized to count 64 cycles of the PLL input clock. During this period, the LOCK
flag is held negated. After the timer expires, the LOCK flag reflects the value coming from the PLL lock detection circuitry. To
prevent an immediate reset, the LOLRE bit must be cleared before doing any of the above operations.
Changing RFD or ERFD does not affect the FMPLL, hence no relock delay is incurred. Resulting changes in clock frequency are
synchronized to the next falling edge of the current system clock. However, RFD or ERFD should only be changed when the
LOCK bit is set, to avoid exceeding the allowable system operating frequency.
Coming out of reset, the FMPLL will be enabled (on), but running in bypass mode. The recommended procedure to program the
FMPLL and engage normal mode is:
1.
Assert the EMODE bit and program the EPREDIV and EMFD fields of FMPLL_ESYNCR1 and the RFD field of
FMPLL_ESYNCR2.
2.
Poll FMPLL_SYNSR[LOCK] until it asserts.
1. Note that changing only the CLKCFG[0] bit to move from bypass to normal or vice-versa, and keeping the values of the other
FMPLL_ESYNCR1 fields unchanged, will not cause the PLL to lose lock or the lock flag to be cleared.
f
VCO
f
sys
f
VCO
4
f
sys
2
RFD
=
f
sys
f
ref
EMFD
EPREDIV
1
+
2
ERFD
1
+
--------------------------------------------------------------------------
=
f
VCO
f
ref
EMFD
EPREDIV
1
+
---------------------------------------
=
Содержание MPC5644A
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