
Clock Generator Module (CGM)
MC68HC08GP32A MC68HC08GP16A Data Sheet, Rev. 1.0
66
Freescale Semiconductor
Also important is the operating voltage potential applied to VDDA. The power supply potential alters the
characteristics of the PLL. A fixed value is best. Variable supplies, such as batteries, are acceptable if
they vary within a known range at very slow speeds. Noise on the power supply is not acceptable,
because it causes small frequency errors which continually change the acquisition time of the PLL.
Temperature and processing also can affect acquisition time because the electrical characteristics of the
PLL change. The part operates as specified as long as these influences stay within the specified limits.
External factors, however, can cause drastic changes in the operation of the PLL. These factors include
noise injected into the PLL through the filter capacitor, filter capacitor leakage, stray impedances on the
circuit board, and even humidity or circuit board contamination.
4.8.3 Choosing a Filter
stability and reaction time of the PLL. The PLL is also dependent on reference frequency and supply
voltage.
Figure 4-11 shows two types of filter circuits. In low-cost applications, where stability and reaction time of
the PLL are not critical, the three component filter network of
Figure 4-11 (B) can be replaced by a single
capacitor, CF, shown in Figure 4-11 (A). Refer to Table 4-4 for recommended filter components at various reference frequencies. For reference frequencies between the values listed in the table, extrapolate to the
nearest common capacitor value. In general, a slightly larger capacitor provides more stability at the
expense of increased lock time.
Figure 4-11. PLL Filter
Table 4-4. Example Filter Component Values
fRCLK
CF1
CF2
RF1
CF
32 kHz
0.15
F
15 nF
2 K
0.22
F
40 kHz
0.12
F
12 nF
2 K
0.18
F
50 kHz
0.10
F
10 nF
2 K
0.18
F
60 kHz
82 nF
8.2 nF
2 K
0.12
F
70 kHz
68 nF
6.8 nF
2 K
0.12
F
80 kHz
56 nF
5.6 nF
2 K
0.1
F
90 kHz
56 nF
5.6 nF
2 K
0.1
F
100 kHz
47 nF
4.7 nF
2 K
82 nF
CGMXFC
RF1
CF2
CF1
VSSA
CGMXFC
CF
VSSA
(A)
(B)