MC68HC908GR8A_07 Datasheet, PDF (73/260 Pages) Freescale Semiconductor, Inc – Microcontrollers

Acquisition/Lock Time Specifications
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 4-10 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 4-10(B) can be replaced by a
single capacitor, C_F, as shown in Figure 4-10(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.
(A) ^CGMXFC
(B) ^CGMXFC
R_F1
C_F
C_F2
C_F1
V_SSA
V_SSA
Figure 4-10. PLL Filter
f_RCLK
1 MHz
2 MHz
3 MHz
4 MHz
5 MHz
6 MHz
7 MHz
8 MHz
Table 4-4. 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
MC68HC908GR8A • MC68HC908GR4A Data Sheet, Rev. 5
Freescale Semiconductor
73