MC68HC908GP32_08 Datasheet, PDF (79/266 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Acquisition/Lock Time Specifications
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.
5.8.3 Choosing a Filter
As described in 5.8.2 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 5-10 is recommended when using a 32.768kHz reference crystal.
Figure 5-10 (a) is used for applications requiring better stability. Figure 5-10 (b) is used in low-cost
applications where stability is not critical.
CGMXFC
CGMXFC
10 kΩ
0.033 µF
0.01 µF
0.47 µF
V_SSA
(a)
V_SSA
(b)
Figure 5-10. PLL Filter
MC68HC908GP32 Data Sheet, Rev. 10
Freescale Semiconductor
79