S12 Clock, Reset and Power Management Unit (S12CPMU)
7.4.5.2
The Adaptive Oscillator Filter
A spike in the oscillator clock can disturb the function of the modules driven by this clock.
The adaptive Oscillator Filter includes two features:
1. Filter noise (spikes) from the incoming external oscillator clock. The filter function is illustrated in
Figure 7-35.
Figure 7-35. Noise filtered by the Adaptive Oscillator Filter
enable external oscillator
OSCE
configure the Oscillator Filter
OSC
0
> 0
FILT
crystal/resonator starts oscillating
EXTAL
LOCK
UPOSC
filtered
filtered
OSCCLK
(filtered)
2. Detect severe noise disturbances on the external oscillator clock, which can not be filtered and
indicate the critical situation to the software by clearing the UPOSC and LOCK status bit and
setting the OSCIF and LOCKIF flag. An example for the detection of critical noise is illustrated in
Figure 7-36.
Figure 7-36. Critical noise detected by the Adaptive Oscillator Filter
enable external oscillator
OSCE
configure the Oscillator Filter
OSC
0
> 0
FILT
crystal/resonator starts oscillating
phase shift can not be filtered but detected
EXTAL
LOCK
UPOSC
OSCCLK
(filtered)
NOTE
If the LOCK bit is clear due to severe noise disturbance on the external
oscillator clock the PLLCLK is derived from the VCO clock (with its actual
frequency) divided by four (see also Section 7.3.2.3, “S12CPMU Post
Divider Register (CPMUPOSTDIV))
S12P-Family Reference Manual, Rev. 1.13
242
Freescale Semiconductor
FREESCALE [ Freescale ]
