NXP Semiconductors MPC5644A, Reference Manual, Page: 578 / 1735

Frequency-modulated phase locked loop (FMPLL)
MPC5644A Microcontroller Reference Manual, Rev. 6
578 Freescale Semiconductor
In bypass mode with crystal reference, a reference fail will force a reset. In bypass mode with external
reference, no backup clock selection occurs if the reference fails.
17.5.4.2 Loss-of-clock reset
When a loss-of-clock condition is recognized, a system reset may be asserted depending on the clock
operating mode and control bits in the FMPLL registers, as shown in
Table 17-13 .
FMPLL_SYNSR[LOCF] and FMPLL_SYNSR[LOC] are cleared after reset, therefore, another means
must be used externally to determine that a loss-of-clock condition occurred.
LOCEN and LOCRE have no effect in bypass mode. If the reference fails while the FMPLL is in bypass
mode with crystal reference, a system reset is asserted regardless of the state of LOCEN and LOCRE.
Since bypass is the FMPLL reset mode, the crystal oscillator must be present and functioning properly to
exit reset when PLLREF = 1. When PLLREF = 0, the reference clock is not checked for loss-of-clock, so
exit from reset can happen regardless the state of the reference clock. Exit from reset is not affected by the
state of the FMPLL output because the FMPLL clock is not monitored in bypass mode.
17.5.4.3 Loss-of-clock interrupt request
When a loss-of-clock condition is recognized, an interrupt request may be asserted depending on the clock
operating mode and control bits in the FMPLL registers, as shown in
Table 17-14 .
LOCEN and LOCIRQ have no effect in bypass mode. If the reference fails in bypass mode with crystal
reference, a system reset is asserted instead of an interrupt request. If the reference fails in bypass with
Table 17-13. Loss-of-clock reset
Operating mode LOCEN
1
1
LOCEN is the loss-of-clock enable bit in either FMPLL_SYNCR or FMPLL_ESYNCR2, depending on
FMPLL_ESYNCR1[EMODE].
LOCRE
2
2
LOCRE is the loss-of-clock reset enable bit in either FMPLL_SYNCR or FMPLL_ESYNCR2, depending on
FMPLL_ESYNCR1[EMODE].
Reset
Reference failure FMPLL failure
Bypass mode with external reference and PLL off — — No No
Bypass mode with crystal reference and PLL off — — Yes No
Bypass mode with external reference and PLL
running — — No No
Bypass mode with crystal reference and PLL running — — Yes No
Normal mode with external reference 0 — No No
1 0 No No
1 1 No Yes
Normal mode with crystal reference 0 — No No
1 0 No No
1 1 Yes Yes