
Clock Generator Module (CGMB)
MC68HC08LK60 MC68HC908LK60 Advance Information Data Sheet, Rev. 1.1
90
Freescale Semiconductor
A value of $00 in the reference divider select register configures the reference divider the same as a
a default divide value of 1.
NOTE
The reference divider select bits have built-in protection such that they
cannot be written when the PLL is on (PLLON = 1).
7.6 Interrupts
When the AUTO bit is set in the PLL bandwidth control register (PBWC), the PLL can generate a CPU
interrupt request every time the LOCK bit changes state. The PLLIE bit in the PLL control register (PCTL)
enables CPU interrupts from the PLL. PLLF, the interrupt flag in the PCTL, becomes set whether
interrupts are enabled or not. When the AUTO bit is clear, CPU interrupts from the PLL are disabled and
PLLF reads as logic 0.
Software should read the LOCK bit after a PLL interrupt request to see if the request was due to an entry
into lock or an exit from lock. When the PLL enters lock, the VCO clock, CGMVCLK, divided by two can
be selected as the CGMOUT source by setting BCS in the PCTL. When the PLL exits lock, the VCO clock
frequency is corrupt, and appropriate precautions should be taken. If the application is not
frequency-sensitive, interrupts should be disabled to prevent PLL interrupt service routines from impeding
software performance or from exceeding stack limitations.
NOTE
Software can select the CGMVCLK divided by two as the CGMOUT source
even if the PLL is not locked (LOCK = 0). Therefore, software should make
sure the PLL is locked before setting the BCS bit.
7.7 Wait Mode
The WAIT instruction put the MCU in low power-consumption standby mode.
The WAIT instruction does not affect the CGMB. Before entering wait mode, software can disengage and
turn off the PLL by clearing the BCS and PLLON bits in the PLL control register (PCTL). Less
power-sensitive applications can disengage the PLL without turning it off. Applications that require the
PLL to wake the MCU from wait mode also can deselect the PLL output without turning off the PLL.
7.8 CGMB During Break Interrupts
The system integration module (SIM) controls whether status bits in other modules can be cleared during
the break state. The BCFE bit in the SIM break flag control register (SBFCR) enables software to clear
To allow software to clear status bits during a break interrupt, write a logic 1 to the BCFE bit. If a status
bit is cleared during the break state, it remains cleared when the MCU exits the break state.
To protect the PLLF bit during the break state, write a logic 0 to the BCFE bit. With BCFE at logic 0 (its
default state), software can read and write the PLL control register during the break state without affecting
the PLLF bit.