MPC563XM Reference Manual, Rev. 1
572
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
17.5.4.1
Alternate/Backup Clock Selection
If loss-of-clock detection is enabled by LOCEN, the FMPLL is operating in normal mode and the Clock
Quality Monitor detects a failure at the FMPLL output clock, then a backup clock selection feature
automatically connects the system clock to the reference clock input (either external or crystal reference).
After this happens, the system clock remains connected to the reference clock until next system reset, even
if the FMPLL regains itself and re-locks. If, however, the reference clock also fails, either simultaneously
or after the FMPLL failure, the system clock is connected back to the FMPLL output.
If the reference fails in normal mode, then no backup clock selection occurs, and the FMPLL output
continues to be the system clock. If the reference stops, the FMPLL will operate in free-running mode.
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
. The LOCF and LOC
bits in SYNSR are cleared after reset, therefore, another means must be used externally to determine that
a loss-of-clock condition occurred.
Normal mode with external reference
0
No
No
1
No
Yes
Normal mode with crystal reference
0
No
No
1
Yes
Yes
1
LOCEN is the loss-of-clock enable bit in either SYNCR or ESYNCR2, depending on the ESYNCR1[EMODE] bit.
Table 17-13. Loss-of-Clock Reset
Operating Mode
LOCEN
1
LOCRE
2
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
Table 17-12. Loss-of-Clock Monitoring (continued)
Operating Mode
LOCEN
1
Reference Clock
Monitored?
FMPLL Output
Monitored?