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VPX 3225D, VPX 3224D
PRELIMINARY DATA SHEET
53
Micronas
4.2.4. Recommended Crystal Characteristics
Symbol
Parameter
Min.
Typ.
Max.
Unit
T
A
Operating Ambient Temperature
0
–
65
°
C
f
P
Parallel Resonance Frequency
with Load Capacitance
C
L
= 13 pF
–
20.250000
fundamental
–
MHz
f
P
/f
P
Accuracy of Adjustment
–
–
±
20
ppm
f
P
/f
P
Frequency Temperature Drift
–
–
±
30
ppm
R
R
Series Resistance
–
–
25
C
0
Shunt Capacitance
3
–
7
pF
C
1
Motional Capacitance
20
–
30
fF
Load Capacitance Recommendation
C
Lext
External Load Capacitance
1
)
from
pins to Ground (PLCC44)
(pin names: Xtal1 Xtal2)
–
4.7
–
pF
DCO Characteristics
2)
C
ICLoadmin
Effective Load Capacitance @ min.
DCO-Position, Code 0,
package: PLCC44
3
4.3
5.5
pF
C
ICLoadrng
Effective Load Capacitance Range,
DCO Codes from 0..255
8.7
12.7
16.7
pF
1)
Remarks on defining the External Load Capacitance:
External capacitors at each crystal pin to ground are required. They are necessary to tune the effective load
capacitance of the PCBs to the required load capacitance (C
L
) of the crystal. The higher the capacitors, the
lower the clock frequency results. The nominal free running frequency should match f
p
= 20.25 MHz. Due to
different layouts of customer PCBs, the matching capacitor size should be determined in the application. The
suggested value is a figure based on experience with various PCB layouts.
Tuning condition: Code DVCO Register = –720
2)
Remarks on Pulling Range of DCO:
The pulling range of the DCO is a function of the used crystal and effective load capacitance of the IC (C
ICLoad
+ C
LoadBoard
). The resulting frequency (f
L
) with an effective load capacitance of C
Leff
= C
ICLoad
+ C
LoadBoard
is
1 + 0.5 * [ C
1
/ (C
0
+ C
L
) ]
f
L
= f
P
* –––––––––––––––––––––––
1 + 0.5 * [ C
1
/ (C
0
+ C
Leff
) ]
3)
Remarks on DCO Codes:
The DCO hardware register has 8 bits; the FP control register uses a range of –2048...2047.