Clock Generator Module (CGM)
Functional Description
MC68HC908AZ60A — Rev 2.0 Technical Data
MOTOROLA Clock Generator Module (CGM) 175
10.4.2.2 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
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 185.
• 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 179. 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 PLL Bandwidth Control Register on page 185. 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 179. 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 189.
