MC68HC708AS48 FREESCALE [Freescale Semiconductor, Inc], MC68HC708AS48 Datasheet - Page 113

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MC68HC708AS48

Manufacturer Part Number
MC68HC708AS48
Description
Advance Information
Manufacturer
FREESCALE [Freescale Semiconductor, Inc]
Datasheet
PLLON — PLL On Bit
This read/write bit activates the PLL and enables the VCO clock,
CGMVCLK. PLLON cannot be cleared if the VCO clock is driving the
base clock, CGMOUT (BCS = 1). (See
Circuit.) Reset sets this bit so that the loop can stabilize as the MCU
is powering up.
BCS — Base Clock Select Bit
This read/write bit selects either the crystal 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
Base Clock Selector
the BCS bit.
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
PCTL[3:0] — Unimplemented bits
These bits provide no function and always read as logic 1s.
MC68HC708AS48
Rev. 4.0
MOTOROLA
1 = PLL on
0 = PLL off
Circuit.) Reset and the STOP instruction clear
1 = CGMVCLK divided by two drives CGMOUT
0 = CGMXCLK divided by two drives CGMOUT
8.4.3 Base Clock Selector Circuit
Clock Generator Module (CGM)
Clock Generator Module (CGM)
CGM Registers
8.4.3 Base Clock Selector
8.4.3
.)
Advance Information
113

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