MC68HC908AT32 Datasheet, PDF (105/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

Interrupts
select bits disables the PLL and clears the BCS bit in the PCTL. (See 8.3.3 Base Clock Selector Circuit
and 8.3.2.5 Special Programming Exceptions for more information.) Reset initializes the bits to $6 to
give a default range multiply value of 6.
NOTE
The VCO range select bits have built-in protection that prevents them from
being written when the PLL is on (PLLON = 1) and prevents selection of the
VCO clock as the source of the base clock (BCS = 1) if the VCO range
select bits are all clear.
The VCO range select bits must be programmed correctly. Incorrect
programming can result in failure of the PLL to achieve lock.
8.6 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 interrupt requests from the PLL. PLLF, the interrupt flag in the PCTL, becomes set whether
CPU interrupt requests are enabled or not. When the AUTO bit is clear, CPU interrupt requests from the
PLL are disabled and PLLF reads as logic 0.
Software should read the LOCK bit after a PLL CPU 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, CPU interrupt requests 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.
8.7 Low-Power Modes
The WAIT and STOP instructions put the MCU in low-power standby modes.
8.7.1 Wait Mode
The CGM remains active in wait mode. 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.
8.7.2 Stop Mode
The STOP instruction disables the CGM and holds low all CGM outputs (CGMXCLK, CGMOUT, and
CGMINT).
If CGMOUT is being driven by CGMVCLK and a STOP instruction is executed, the PLL will clear the BCS
bit in the PLL control register, causing CGMOUT to be driven by CGMXCLK. When the MCU recovers
from STOP, the crystal clock divided by two drives CGMOUT and BCS remains clear.
MC68HC908AT32 Data Sheet, Rev. 3.1
Freescale Semiconductor
105