MC9S12DT256 Device User Guide — V03.07
113
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-3 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
.
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:
The phase detector relationship is given by:
i
ch
is the current in tracking mode.
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
f
1
f
vco
–
(
)
K
1
1V
⋅
-----------------------
⋅
=
100
–
e
60
50
–
(
)
100
–
------------------------
⋅
=
= -90.48MHz/V
K
Φ
i
ch
–
K
V
⋅
=
=
316.7Hz/
Ω
Содержание MC9S12A256
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