MC68HC908GP32_08 Datasheet, PDF (64/266 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Clock Generator Module (CGM)
The phase detector then compares the VCO feedback clock, CGMVDV, with the final reference clock,
CGMRDV. A correction pulse is generated based on the phase difference between the two signals. The
loop filter then slightly alters the DC voltage on the external capacitor connected to CGM/XFC based on
the width and direction of the correction pulse. The filter can make fast or slow corrections depending on
its mode, described in 5.3.4 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.
5.3.4 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 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 5.5.2 PLL Bandwidth Control Register.)
• 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 5.3.8 Base Clock Selector Circuit.) The PLL is automatically in
tracking mode when not in acquisition mode or when the ACQ bit is set.
5.3.5 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 5.5.2 PLL
Bandwidth Control Register.) If PLL interrupts are enabled, the software can wait for a PLL interrupt
request and then check the LOCK bit. If 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 5.3.8 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 5.6
Interrupts for information and precautions on using interrupts.)
The following conditions apply when the PLL is in automatic bandwidth control mode:
• The ACQ bit (See 5.5.2 PLL Bandwidth Control Register.) is a read-only indicator of the mode of
the filter. (See 5.3.4 Acquisition and Tracking Modes.)
• The ACQ bit is set when the VCO frequency is within a certain tolerance and is cleared when the
VCO frequency is out of a certain tolerance. (See 5.8 Acquisition/Lock Time Specifications for
more information.)
• The LOCK bit is a read-only indicator of the locked state of the PLL.
MC68HC908GP32 Data Sheet, Rev. 10
64
Freescale Semiconductor