MC68HC908AZ60 Datasheet, PDF (135/480 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Clock Generator Module (CGM)
Functional Description
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 PLL Bandwidth Control Register on page 143.
• 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 Base Clock Selector Circuit on page 137. The PLL is
automatically in tracking mode when it’s not in acquisition mode or
when the ACQ bit is set.
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 PLL Bandwidth Control Register on page 143. 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 Base Clock Selector Circuit on page 137. 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 Interrupts on page 147.
These conditions apply when the PLL is in automatic bandwidth control
mode:
7-cgm
MOTOROLA
Clock Generator Module (CGM)
MC68HC908AZ60 — Rev 2.0
133