AN2727 Freescale Semiconductor / Motorola, AN2727 Datasheet - Page 14

• Manufacturer Part Number: AN2727
• Description: Designing Hardware for the HCS12 D-Family
• Manufacturer: Freescale Semiconductor / Motorola

HCS12 PLL
HCS12 PLL
PLL Filter Circuit
The HCS12 PLL allows programmable bus frequencies to be generated from the oscillator clock.
With the PLL disabled:
With the PLL enabled:
The PLL on the HCS12 requires a three-component passive filter to be connected to the XFC pin, as in
Figure
filter are contained in the device user guide, but it is simpler to use the PLL filter calculator from Freescale.
If, for some reason, it is decided not to fit a filter network, the XFC pin should be connected via a pull-up
resistor (1k to 5k) to VDDPLL.
If the application does not use the PLL, the software should clear the PLLON bit to disable the PLL
following reset.
14
4. The values of the filter components are application specific. The equations for calculating the
ECLK (bus) freq
Designing Hardware for the HCS12 D-Family, Rev. 0
ECLK (bus) freq
Never connect XFC to VSSPLL.
MCU
XFC
=
Figure 4. XFC Circuit
Oscillator (Crystal) Freq
=
CAUTION
Oscillator (Crystal) Freq
R
C
S
s
¥
VDDPLL
------------------------------------- -
(
C
REFDIV
(
P
SYNR
§
2
+
+
1
1
)
)
Freescale Semiconductor