
107
A.5.3 Phase Locked Loop
The oscillator provides the reference clock for the PLL. The PLL′s Voltage Controlled Oscillator (VCO)
is also the system clock source in self clock mode.
A.5.3.1 XFC Component Selection
This section describes the selection of the XFC components to achieve a good filter characteristics.
Figure A-2 Basic PLL functional diagram
The following procedure can be used to calculate the resistance and capacitance values using typical
values for K
1
, f
1
and i
ch
from
Table A-16
.
The grey boxes show the calculation for f
VCO
= 50MHz and f
ref
= 1MHz. E.g., these frequencies are used
for f
OSC
= 4MHz and a 25MHz bus clock.
The VCO Gain at the desired VCO frequency is approximated by:
f
f
–
(
)
K
1
1V
=
100
–
=
NOTES
:
1. Depending on the crystal a damping series resistor might be necessary
2. f
osc
= 4MHz, C = 22pF.
3. Maximum value is for extreme cases using high Q, low frequency crystals
4. Only valid if Pierce oscillator/external clock mode is selected
f
osc
1
refdv+1
f
ref
Phase
Detector
VCO
K
V
1
synr+1
f
vco
Loop Divider
K
Φ
1
2
f
cmp
C
s
R
C
p
VDDPLL
XFC Pin
K
V
K
1
e
-----------------------
e
60
–
---------------50
)
100
= -90.48MHz/V
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.