MC908GR16ACFA FREESCALE [Freescale Semiconductor, Inc], MC908GR16ACFA Datasheet - Page 71

• Manufacturer Part Number: MC908GR16ACFA
• Description: M68HC08 Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

VRS7–VRS0 — VCO Range Select Bits
4.6 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 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.
4.7 Special Modes
The WAIT instruction puts the MCU in low power-consumption standby modes.
4.7.1 Wait Mode
The WAIT instruction does not affect the CGM. 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) to save power.
Less power-sensitive applications can disengage the PLL without turning it off, so that the PLL clock is
immediately available at WAIT exit. This would be the case also when the PLL is to wake the MCU from
wait mode, such as when the PLL is first enabled and waiting for LOCK or LOCK is lost.
Freescale Semiconductor
These read/write bits control the hardware center-of-range linear multiplier L which, in conjunction with
E (See
hardware center-of-range frequency, f
PCTL is set. (See
register disables the PLL and clears the BCS bit in the PLL control register (PCTL). (See
Clock Selector Circuit
for a default range multiply value of 64.
4.3.3 PLL
The VCO range select bits have built-in protection such that they cannot be
written when the PLL is on (PLLON = 1) and such that the VCO clock
cannot be selected as the source of the base clock (BCS = 1) if the VCO
range select bits are all clear.
The PLL VCO range select register must be programmed correctly.
Incorrect programming can result in failure of the PLL to achieve lock.
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.
Circuits,
4.3.7 Special Programming
and
4.3.7 Special Programming
4.3.6 Programming the
MC68HC908GR16A Data Sheet, Rev. 1.0
VRS
. VRS7–VRS0 cannot be written when the PLLON bit in the
NOTE
NOTE
Exceptions.) A value of $00 in the VCO range select
PLL, and
Exceptions.). Reset initializes the register to $40
4.5.1 PLL Control
Register.), controls the
4.3.8 Base
Interrupts
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