Clock Generator Module (CGM)
Functional Description
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA Clock Generator Module (CGM) 119
8.4.2.5 Special Programming Exceptions
The programming method described in 8.4.2.4 Programming the PLL
does not account for possible exceptions. A value of 0 for N or L is
meaningless when used in the equations given. To account for these
exceptions:
• A 0 value for N is interpreted exactly the same as a value of 1.
• A 0 value for L disables the PLL and prevents its se le cti on as the
source for the base clock. See 8.4.3 Base Clock Selector
Circuit.
8.4.3 Base Clock Selector Circuit
This circuit is used to select either the crystal clock, CGMXCLK, or the
VCO clock, CGMVCLK, as the source of the base clock, CGMOUT. The
two input clocks go through a transition control circuit that waits up to
three CGMXCLK cycles and three CGMVCLK cycles to change from
one clock source to the other. During this time, CGMOUT is held in
stasis. The output of the transition control circuit is then divided by two
to correct the duty cycle. Therefore, the bus clock frequency, which is
one-half of the base clock frequency, is one-fourth the frequency of the
selected clock (CGMXCLK or CGMVCLK).
The BCS bit in the PLL control register (PCTL) selects which clock drives
CGMOUT. The VCO clock cannot be selected as the base clock source
if the PLL is not turned on. The PLL cannot be turned off if the VCO clock
is selected. The PLL cannot be turned on or off simultaneously with the
selection or deselection of the VCO clock. The VCO clock also cannot
be selected as the base clock source if the factor L is programmed to a0.
This value would set up a condition inconsistent with the operati on of the
PLL, so that the PLL would be disabled and the crystal clock would be
forced as the source of the base clock.
8.4.4 CGM External Connections
In its typical configuration, the CGM requires seven external
components. Five of these are for the crystal oscillator and two are for
the PLL.