MC68HC908GZ8 Datasheet, PDF (100/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
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.
7.7.2 Stop Mode
If the OSCSTOPENB bit in the CONFIG register is cleared (default), then the
STOP instruction disables the CGM (oscillator and phase locked loop) and holds
low all CGM outputs (CGMXCLK, CGMOUT, and CGMINT).
If the OSCSTOPENB bit in the CONFIG register is set, then the phase locked loop
is shut off but the oscillator will continue to operate in stop mode.
7.7.3 CGM 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 status bits during the break state. (See
20.7.3 Break Flag Control Register.)
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.
7.8 Acquisition/Lock Time Specifications
The acquisition and lock times of the PLL are, in many applications, the most
critical PLL design parameters. Proper design and use of the PLL ensures the
highest stability and lowest acquisition/lock times.
7.8.1 Acquisition/Lock Time Definitions
Typical control systems refer to the acquisition time or lock time as the reaction
time, within specified tolerances, of the system to a step input. In a PLL, the step
input occurs when the PLL is turned on or when it suffers a noise hit. The tolerance
is usually specified as a percent of the step input or when the output settles to the
desired value plus or minus a percent of the frequency change. Therefore, the
reaction time is constant in this definition, regardless of the size of the step input.
For example, consider a system with a 5 percent acquisition time tolerance. If a
command instructs the system to change from 0 Hz to 1 MHz, the acquisition time
is the time taken for the frequency to reach 1 MHz ±50 kHz. Fifty kHz = 5% of the
1-MHz step input. If the system is operating at 1 MHz and suffers a –100-kHz noise
Data Sheet
100
Clock Generator Module (CGM)
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MC68HC908GZ8
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