MC908GZ60CFUE Datasheet, PDF (90/352 Pages) Freescale Semiconductor, Inc – Standard features, Features of the CPU08

Clock Generator Module (CGM)
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 4-9 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 shown in Figure 4-9 (B) can be replaced by a
single capacitor, C_F, as shown in shown in Figure 4-9 (A). Refer to Table 4-5 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 4-9. PLL Filter
f_RCLK
1 MHz
2 MHz
3 MHz
4 MHz
5 MHz
6 MHz
7 MHz
8 MHz
Table 4-5. Example Filter Component Values
C_F1
8.2 nF
4.7 nF
3.3 nF
2.2 nF
1.8 nF
1.5 nF
1.2 nF
1 nF
C_F2
R_F1
820 pF
2k
470 pF
2k
330 pF
2k
220 pF
2k
180 pF
2k
150 pF
2k
120 pF
2k
100 pF
2k
C_F
18 nF
6.8 nF
5.6 nF
4.7 nF
3.9 nF
3.3 nF
2.7 nF
2.2 nF
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
90
Freescale Semiconductor