S12 Clock, Reset and Power Management Unit (S12CPMU)
The use of the filter function is only possible if the VCOCLK-to-OSCCLK ratio divided by two ((f
/
VCO
f
)/2) is an integer number. This integer value must be written to the OSCFILT[4:0] bits.
OSC
If enabled, the oscillator filter is sampling the incoming oscillator clock signal (EXTAL) with the
VCOCLK frequency.
Using VCOCLK, a time window is defined during which an edge of the OSCCLK is expected. In case of
OSCBW = 1 the width of this window is three VCOCLK cycles, if the OSCBW = 0 it is one VCOCLK
cycle.
The noise detection is active for certain combinations of OSCFILT[4:0] and OSCBW bit settings as shown
in Table 7-24
Table 7-24. Noise Detection Settings
OSCFILT[4:0]
OSCBW
Detection
Filter
0
1
x
x
0
1
x
disabled
disabled
active
disabled
active
active
active
active
2 or 3
disabled
active
>=4
NOTE
If the VCOCLK frequency is higher than 25 MHz the wide bandwidth must
be selected (OSCBW = 1).
S12P-Family Reference Manual, Rev. 1.13
Freescale Semiconductor
243
FREESCALE [ Freescale ]
