Clock Generator Module (CGM)
MC68HC08GZ32 Data Sheet, Rev. 3
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Freescale Semiconductor
4.3.4 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
VCO frequency is far off the desired frequency. When in acquisition mode, the ACQ bit is clear in
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
tracking mode when not in acquisition mode or when the ACQ bit is set.
4.3.5 Manual and Automatic PLL Bandwidth Modes
The PLL can change the bandwidth or operational mode of the loop filter manually or automatically.
Automatic mode is recommended for most users.
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
4.5.2 PLLrequest and then check the LOCK bit. If interrupts are disabled, software can poll the LOCK bit
continuously (for example, during PLL start up) or at periodic intervals. In either case, when the LOCK bit
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
4.6 Interrupts for information and precautions on using interrupts.)
The following conditions apply when the PLL is in automatic bandwidth control mode:
The ACQ bit is set when the VCO frequency is within a certain tolerance and is cleared when the
more information.)
The LOCK bit is a read-only indicator of the locked state of the PLL.
The LOCK bit is set when the VCO frequency is within a certain tolerance and is cleared when the
more information.)
CPU interrupts can occur if enabled (PLLIE = 1) when the PLL’s lock condition changes, toggling
The PLL also may operate in manual mode (AUTO = 0). Manual mode is used by systems that do not
require an indicator of the lock condition for proper operation. Such systems typically operate well below
fBUSMAX.