Clock Generator Module (CGM)
NOTE
PLLON and BCS have built-in protection that prevents the base clock
selector circuit from selecting the VCO clock as the source of the base clock
if the PLL is off. Therefore, PLLON cannot be cleared when BCS is set, and
BCS cannot be set when PLLON is clear. If the PLL is off (PLLON = 0),
selecting CGMPCLK requires two writes to the PLL control register. (See
6.3.8 Base Clock Selector Circuit.)
PRE1 and PRE0 — Prescaler Program Bits
These read/write bits control a prescaler that selects the prescaler power-of-two multiplier, P. (See
6.3.3 PLL Circuits and 6.3.6 Programming the PLL.) PRE1 and PRE0 cannot be written when the
PLLON bit is set. Reset clears these bits.
These prescaler bits affects the relationship between the VCO clock and the final system bus clock.
Table 6-2. PRE1 and PRE0 Programming
PRE1 and PRE0
P
0
1
2
3
Prescaler Multiplier
00
01
10
11
1
2
4
8
VPR1 and VPR0 — VCO Power-of-Two Range Select Bits
These read/write bits control the VCO’s hardware power-of-two range multiplier E that, in conjunction
with L (See 6.3.3 PLL Circuits, 6.3.6 Programming the PLL, and 6.5.4 PLL VCO Range Select
Register.) controls the hardware center-of-range frequency, f
the PLLON bit is set. Reset clears these bits.
. VPR1:VPR0 cannot be written when
VRS
Table 6-3. VPR1 and VPR0 Programming
VCO Power-of-Two
Range Multiplier
VPR1 and VPR0
E
00
0
1
2
1
2
4
01
10
NOTE: Do not program E to a value of 3.
MC68HC908AP Family Data Sheet, Rev. 4
90
Freescale Semiconductor
FREESCALE [ Freescale ]
