Clock Generator Module (CGM)
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.
6.8.3 Choosing a Filter
As described in 6.8.2 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.
Either of the filter networks in Figure 6-10 is recommended when using a 32.768kHz reference clock
(CGMRCLK). Figure 6-10 (a) is used for applications requiring better stability. Figure 6-10 (b) is used in
low-cost applications where stability is not critical.
CGMXFC
CGMXFC
1 kΩ
10 nF
0.22 µF
0.22 µF
V
SSA
V
SSA
(a)
(b)
Figure 6-10. PLL Filter
MC68HC908AP Family Data Sheet, Rev. 4
96
Freescale Semiconductor
FREESCALE [ Freescale ]
