MC68HC908GZ8 Datasheet, PDF (102/344 Pages) Motorola, Inc – Microcontrollers

Clock Generator Module (CGM)
7.8.3 Choosing a Filter
As described in 7.8.2 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 7-9 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 7-9 (B) can be replaced by a single capacitor, C_F, as shown in
shown in Figure 7-9 (A). Refer to Table 7-5 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
C_F
R_F1
C_F2
C_F1
V_SSA
V_SSA
(A)
(B)
Figure 7-9. PLL Filter
f_RCLK
1 MHz
2 MHz
3 MHz
4 MHz
5 MHz
6 MHz
7 MHz
8 MHz
Table 7-5. 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
Data Sheet
102
Clock Generator Module (CGM)
MC68HC908GZ8
Freescale Semiconductor