MC908AZ32ACFUE Datasheet, PDF (100/324 Pages) Freescale Semiconductor, Inc – Microcontrollers

Clock Generator Module (CGM)
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 8.3.3 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 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 8.3.3 Base Clock
Selector Circuit. Reset and the STOP instruction clear 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
8.3.3 Base Clock Selector Circuit.
PCTL3–PCTL0 — Unimplemented
These bits provide no function and always read as logic 1s.
8.5.2 PLL Bandwidth Control Register
The PLL bandwidth control register:
• Selects automatic or manual (software-controlled) bandwidth control mode
• Indicates when the PLL is locked
• In automatic bandwidth control mode, indicates when the PLL is in acquisition or tracking mode
• In manual operation, forces the PLL into acquisition or tracking mode
Address: $001D
Bit 7
6
5
4
3
2
1
Bit 0
Read:
LOCK
0
0
0
0
AUTO
ACQ
XLD
Write:
Reset: 0
0
0
0
0
0
0
0
= Unimplemented
Figure 8-5. PLL Bandwidth Control Register (PBWC)
AUTO — Automatic Bandwidth Control Bit
This read/write bit selects automatic or manual bandwidth control. When initializing the PLL for manual
operation (AUTO = 0), clear the ACQ bit before turning on the PLL. Reset clears the AUTO bit.
1 = Automatic bandwidth control
0 = Manual bandwidth control
MC68HC908AZ32A Data Sheet, Rev. 2
100
Freescale Semiconductor