Datasheet

Functional Description
MC68HC908GR16A Data Sheet, Rev. 1.0
Freescale Semiconductor 63
Table 4-3 provides numeric examples (register values are in hexadecimal notation):
4.3.7 Special Programming Exceptions
The programming method described in 4.3.6 Programming the PLL does not account for two 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 selection as the source for the base clock.
See 4.3.8 Base Clock Selector Circuit.
4.3.8 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 a 0. This value would set up a condition inconsistent with the operation of the
PLL, so that the PLL would be disabled and the crystal clock would be forced as the source of the base
clock.
Table 4-3. Numeric Example
f
BUS
(MHz) f
RCLK
(MHz)
PCTL
E
PMSH,L
N
PMRS
L
1.0 2.0 0 002 38
2.0 2.0 0 004 70
4.0 2.0 1 008 70
8.0 2.0 2 010 70
2.0 4.0 0 002 70
4.0 4.0 1 004 70
5.0 4.0 2 005 46
8.0 4.0 2 008 70
2.4576 4.9152 1 002 45
4.9152 4.9152 2 004 45
7.3728 4.9152 2 006 67
2.0 8.0 0 001 70
4.0 8.0 1 002 70
6.0 8.0 2 003 54
8.0 8.0 2 004 70