MC68HC08GP32A Datasheet, PDF (66/258 Pages) Freescale Semiconductor, Inc – Microcontrollers

Clock Generator Module (CGM)
Also important is the operating voltage potential applied to V_DDA. 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
As described in 4.8.2 Parametric Influences on Reaction Time, the external filter network is critical to the
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, C_F, 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.
CGMXFC
CGMXFC
R_F1
C_F
C_F2
C_F1
V_SSA
V_SSA
(A)
(B)
Figure 4-11. PLL Filter
Table 4-4. Example Filter Component Values
f_RCLK
32 kHz
40 kHz
50 kHz
60 kHz
70 kHz
80 kHz
90 kHz
100 kHz
C_F1
0.15 µF
0.12 µF
0.10 µF
82 nF
68 nF
56 nF
56 nF
47 nF
C_F2
R_F1
15 nF
2K
12 nF
2K
10 nF
2K
8.2 nF
2K
6.8 nF
2K
5.6 nF
2K
5.6 nF
2K
4.7 nF
2K
C_F
0.22 µF
0.18 µF
0.18 µF
0.12 µF
0.12 µF
0.1 µF
0.1 µF
82 nF
MC68HC08GP32A • MC68HC08GP16A Data Sheet, Rev. 1.0
66
Freescale Semiconductor