MC68HC908JW32_09 Datasheet, PDF (68/232 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Clock Generator Module (CGM)
BCS — Base Clock Select Bit
This read/write bit selects either the oscillator output, CGMXCLK, or the VCO clock, CGMVCLK, as
the source of the CGM output, CGMOUT. CGMOUT frequency is one-half the frequency of the
selected clock. BCS cannot be set while the PLLON bit is clear. After toggling BCS, it may take up to
three CGMXCLK and three CGMVCLK cycles to complete the transition from one source clock to the
other. During the transition, CGMOUT is held in stasis. (See 5.3.8 Base Clock Selector Circuit.) Reset
clears the BCS bit.
1 = CGMVCLK divided by three drives CGMOUT
0 = CGMXCLK divided by two drives CGMOUT
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 CGMVCLK requires two writes to the PLL control register. (See
5.3.8 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
5.3.3 PLL Circuits and 5.3.6 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 5-2. PRE1 and PRE0 Programming
PRE1 and PRE0
00
01
10
11
P
Prescaler Multiplier
0
1
1
2
2
4
3
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 5.3.3 PLL Circuits, 5.3.6 Programming the PLL, and 5.5.4 PLL VCO Range Select
Register.) controls the hardware center-of-range frequency, f_VRS. VPR1:VPR0 cannot be written when
the PLLON bit is set. Reset clears these bits.
Table 5-3. VPR1 and VPR0 Programming
VPR1 and VPR0
E
00
0
01
1
10
2
NOTE: Do not program E to a value of 3.
VCO Power-of-Two
Range Multiplier
1
2
4
MC68HC908JW32 Data Sheet, Rev. 6
68
Freescale Semiconductor