MC68HC08AZ32A Datasheet, PDF (139/456 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
Acquisition/Lock Time Specifications
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% 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 hit, the acquisition time
is the time taken to return from 900 kHz to 1 MHz ±5 kHz. Five kHz = 5%
of the 100-kHz step input.
Other systems refer to acquisition and lock times as the time the system
takes to reduce the error between the actual output and the desired
output to within specified tolerances. Therefore, the acquisition or lock
time varies according to the original error in the output. Minor errors may
not even be registered. Typical PLL applications prefer to use this
definition because the system requires the output frequency to be within
a certain tolerance of the desired frequency regardless of the size of the
initial error.
The discrepancy in these definitions makes it difficult to specify an
acquisition or lock time for a typical PLL. Therefore, the definitions for
acquisition and lock times for this module are:
• Acquisition time, t_acq, is the time the PLL takes to reduce the error
between the actual output frequency and the desired output
frequency to less than the tracking mode entry tolerance, ∆_trk.
Acquisition time is based on an initial frequency error,
(f_des – f_orig)/f_des, of not more than ±100%. In automatic bandwidth
control mode (see Manual and Automatic PLL Bandwidth
Modes on page 123), acquisition time expires when the ACQ bit
becomes set in the PLL bandwidth control register (PBWC).
• Lock time, t_Lock, is the time the PLL takes to reduce the error
between the actual output frequency and the desired output
frequency to less than the lock mode entry tolerance, ∆_Lock. Lock
time is based on an initial frequency error, (f_des – f_orig)/f_des, of not
more than ±100%. In automatic bandwidth control mode, lock time
MC68HC08AZ32A — Rev 1.0
MOTOROLA
Clock Generator Module (CGM)
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Technical Data
139