Datasheet

Clock Generator Module (CGM)
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
90 Freescale Semiconductor
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.
Figure 4-9. PLL Filter
Table 4-5. Example Filter Component Values
f
RCLK
C
F1
C
F2
R
F1
C
F
1 MHz 8.2 nF 820 pF 2k 18 nF
2 MHz 4.7 nF 470 pF 2k 6.8 nF
3 MHz 3.3 nF 330 pF 2k 5.6 nF
4 MHz 2.2 nF 220 pF 2k 4.7 nF
5 MHz 1.8 nF 180 pF 2k 3.9 nF
6 MHz 1.5 nF 150 pF 2k 3.3 nF
7 MHz 1.2 nF 120 pF 2k 2.7 nF
8 MHz 1 nF 100 pF 2k 2.2 nF
CGMXFC
R
F1
C
F2
C
F1
V
SSA
CGMXFC
C
F
V
SSA
(A) (B)