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MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-17
The PLL loop filter has two bandwidths which are automatically selected. When the
PLL is first enabled, the wide bandwidth mode is used which enables the PLL fre-
quency to ramp up quickly. Then when the output frequency is near the desired
frequency, the filter is switched to the narrow bandwidth to make the final frequency
more stable. The PLL requires an external filter network on XFC. Figure 4-5 shows
the suggested values of bypass and PLL external capacitors.
For slow reference mode with a multiplication factor of N=512 (ex: 32.768 KHz refer-
ence for a 16.78 MHz fSYS) the values are: R1 = 18 K; C1 = 0.1 F; and C2 = 3300 pF.
For fast reference mode with a multiplication factor of N=4 (ex: 4.194 MHz reference
and 16.78 MHz fSYS) the values are: R1 = 20 K; C1 = 1000 pF; and C2 = 100 pF. Ref-
erence frequencies and fSYS values very different from those stated may require
different filter values (first order affect is multiplication factor N, based on W and Y bits
in the SYCNR register). Leakage from the XFC pin must not be greater than that of a
15-m
resistor to meet jitter specifications. If the PLL is not enabled, then the XFC fil-
ter is not required and the pin may be left unconnected.
The VCO will oscillate at a frequency dictated by the voltage coming out of the filter
circuit. To save system power, the VCO is shut off when it is not needed.
Figure 4-5 Crystal Oscillator and External Capacitor Configuration
XFC Pin
VSSSYN Pin
VSSSYN
EXTAL PIN
XTAL PIN
Crystal
C2
C1
RF
Slow Reference Mode (32.768 KHz): RF=1-10 m
RS=330 K
, C1=C2=22 pF
Fast Reference Mode (4.194 MHz):RF=1-10 m
RS=1.5 K
, C1=C2=27 pF
Fast Reference Mode (5.438 MHz):RF=1-10 m
RS=400
, C1=C2=10 pF
Location of RS can be inboard (as shown here) or
outboard of the crystal.
NOTE: Actual circuit should be reviewed with crystal
manufacturer.
VSSSYN
RS
C1
C2
R1
Slow Reference Mode (32.768 KHz):
,
C1=0.1
F, C2=3300 pF
Fast Reference Mode (4.194 - 5.438 MHz):
,
C1=4000 pF, C2=330 pF
R1
R
0
Y1
+
() 2
2W
1
+
()
R
0
;
475.2
==
R1
R
0
W1
+
() R
0
1197
=
;
=
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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