MC68HC908GP32 Datasheet, PDF (126/410 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

Freescale Semiconductor, Inc.
Clock Generator Module (CGMC)
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 7.4.8 Base Clock Selector
Circuit.) Reset sets this bit so that the loop can stabilize as the MCU
is powering up.
1 = PLL on
0 = PLL off
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
CGMC 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 7.4.8 Base Clock Selector Circuit.) Reset clears the BCS bit.
1 = CGMVCLK divided by two 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 7.4.8 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 7.4.3 PLL Circuits and 7.4.6
Programming the PLL.) PRE1 and PRE0 cannot be written when
the PLLON bit is set. Reset clears these bits.
NOTE: The value of P is normally 0 when using a 32.768-kHz crystal as the
reference.
Technical Data
124
MC68HC908GP32•MC68HC08GP32 — Rev. 6
Clock Generator Module (CGMC)
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