Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
Functional Description
10.4.2.2 Acquisition and Tracking Modes
The PLL filter is manually or automatically configurable into one of two
operating modes:
1. Acquisition mode — In acquisition mode, the filter can make large
frequency corrections to the VCO. This mode is used at PLL
startup or when the PLL has suffered a severe noise hit and the
VCO frequency is far off the desired frequency. When in
acquisition mode, the ACQ bit is clear in the PLL bandwidth control
register. See 10.6.2 PLL Bandwidth Control Register.
2. 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 10.4.3 Base Clock Selector Circuit. The PLL is
automatically in tracking mode when it’s not in acquisition mode or
when the ACQ bit is set.
10.4.2.3 Manual and Automatic PLL Bandwidth Modes
The PLL can change the bandwidth or operational mode of the loop filter
manually or automatically.
In automatic bandwidth control mode (AUTO = 1), the lock detector
automatically switches between acquisition and tracking modes.
Automatic bandwidth control mode also is used to determine when the
VCO clock, CGMVCLK, is safe to use as the source for the base clock,
CGMOUT. See 10.6.2 PLL Bandwidth Control Register. If PLL CPU
interrupt requests are enabled, the software can wait for a PLL CPU
interrupt request and then check the LOCK bit. If CPU interrupts are
disabled, software can poll the LOCK bit continuously (during PLL
startup, usually) or at periodic intervals. In either case, when the LOCK
bit is set, the VCO clock is safe to use as the source for the base clock.
See 10.4.3 Base Clock Selector Circuit. If the VCO is selected as the
source for the base clock and the LOCK bit is clear, the PLL has suffered
a severe noise hit and the software must take appropriate action,
depending on the application. See 10.7 Interrupts.
MC68HC908AS60 — Rev. 1.0
Technical Data
Clock Generator Module (CGM)
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