CGM Registers
6.5.1 PLL Control Register
The PLL control register (PCTL) contains the interrupt enable and flag bits, the on/off switch, the base
clock selector bit, the prescaler bits, and the VCO power-of-two range selector bits.
Address:
$0036
Bit 7
6
5
PLLON
1
4
BCS
0
3
PRE1
0
2
PRE0
0
1
VPR1
0
Bit 0
VPR0
0
Read:
Write:
Reset:
PLLF
PLLIE
0
0
= Unimplemented
Figure 6-4. PLL Control Register (PCTL)
PLLIE — PLL Interrupt Enable Bit
This read/write bit enables the PLL to generate an interrupt request when the LOCK bit toggles, setting
the PLL flag, PLLF. When the AUTO bit in the PLL bandwidth control register (PBWC) is clear, PLLIE
cannot be written and reads as logic 0. Reset clears the PLLIE bit.
1 = PLL interrupts enabled
0 = PLL interrupts disabled
PLLF — PLL Interrupt Flag Bit
This read-only bit is set whenever the LOCK bit toggles. PLLF generates an interrupt request if the
PLLIE bit also is set. PLLF always reads as logic 0 when the AUTO bit in the PLL bandwidth control
register (PBWC) is clear. Clear the PLLF bit by reading the PLL control register. Reset clears the PLLF
bit.
1 = Change in lock condition
0 = No change in lock condition
NOTE
Do not inadvertently clear the PLLF bit. Any read or read-modify-write
operation on the PLL control register clears the PLLF bit.
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 6.3.8 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 oscillator output, CGMXCLK, or the divided VCO clock,
CGMPCLK, 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 CGMPCLK cycles to complete the transition from one
source clock to the other. During the transition, CGMOUT is held in stasis. (See 6.3.8 Base Clock
Selector Circuit.) Reset clears the BCS bit.
1 = CGMPCLK divided by two drives CGMOUT
0 = CGMXCLK divided by two drives CGMOUT
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
89
FREESCALE [ Freescale ]
