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MPC563XM Reference Manual, Rev. 1
Freescale Semiconductor
571
Preliminary—Subject to Change Without Notice
the external clock generator when using external reference mode. The FMPLL output may only monitored
in normal mode, depending on the state of the LOCEN bit.
The clock quality monitor uses an internal 4 MHz RC oscillator as a reference time base to measure the
frequency of the crystal oscillator and the FMPLL output. The frequency of these clocks are expected to
be within the following frequency ranges:
•
4 MHz to 20 MHz for the crystal clock
•
Above 1.5 MHz for the PLL output (minimum VCO free-running frequency divided by the
maximum ERFD)
The clocks are considered to be of quality when their frequencies stay within the limits for 3 consecutive
measuring cycles. In the event either of the clocks fall outside the expected window, a loss of clock
condition is reported. The FMPLL can be programmed to switch the system clock to a backup clock in the
event of such a failure. Additionally, the user may select to have the system enter reset, assert an interrupt
request, or do nothing if/when the FMPLL reports this condition.
Figure 17-7. Clock Quality Monitor
Table 17-12. Loss-of-Clock Monitoring
Operating Mode
LOCEN
1
Reference Clock
Monitored?
FMPLL Output
Monitored?
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
RC OSC
4 MHz
CRYSTAL OSC
4 - 20 MHz
> 1.5 MHz
COUNTER 1
COUNTER 2
COUNTER 3
PLL
CONTROL
LOGIC