Clock Generator Module (CGM)
Technical Data MC68HC908AB32 — Rev. 1.0
136 Clock Generator Module (CGM) MOTOROLA
then slightly alters the DC voltage on the external capacitor connected
to CGMXFC based on the width and direction of the correction pulse.
The filter can make fast or slow corrections depending on its mode,
described in 9.4.2.2 Acquisition and Tracking Modes. The value of the
external capacitor and the reference frequency determines the speed of
the corrections and the stability of the PLL.
The lock detector compares the frequencies of the VCO feedback clock,
CGMVDV, and the final reference clock, CGMRDV. Therefore, the
speed of the lock detector is directly proportional to the final reference
frequency f
RDV
. The circuit determines the mode of the PLL and the lock
condition based on this comparison.
9.4.2.2 Acquisition and Tracking Modes
The PLL filter is manually or automatically configurable into one of two
operating modes:
• Acquisition mode — in acquisition mode, the filter can make large
frequency corrections to the VCO. This mode is used at PLL start-
up or when the PLL has suffered a severe noise hit and the
resulting VCO frequency is much different from the desired
frequency. When in acquisition mode, the ACQ
bit is clear in the
PLL bandwidth control register. See 9.6.2 PLL Bandwidth
Control Register (PBWC).
• Tracking mode — in tracking mode, the filter makes only small
corrections to the frequency of the VCO. PLL jitter is much lower
in tracking mode, but the response to noise is also slower. The
PLL enters tracking mode when the VCO frequency is nearly
correct, such as when the PLL is selected as the base clock
source. See 9.4.3 Base Clock Selector Circuit. The PLL is
automatically in tracking mode when not in acquisition mode or
when the ACQ
bit is set.
9.4.2.3 Manual and Automatic PLL Bandwidth Modes
The PLL can change the bandwidth or operational mode of the loop filter
manually or automatically.
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