MC68HC08AZ32A Datasheet, PDF (76/312 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Clock Generator Module (CGM)
multiple of n_ACQ/f_CGMRDV, and the acquisition to lock time, t_AL, is an integer multiple of n_TRK/f_CGMRDV.
Also, since the average frequency over the entire measurement period must be within the specified
tolerance, the total time usually is longer than t_Lock as calculated above.
In manual mode, it is usually necessary to wait considerably longer than t_Lock before selecting the PLL
clock (see 4.3.3 Base Clock Selector Circuit), because the factors described in 4.9.2 Parametric
Influences on Reaction Time, may slow the lock time considerably.
When defining a limit in software for the maximum lock time, the value must allow for variation due to all
of the factors mentioned in this section, especially due to the C_F capacitor and application specific
influences.
The calculated lock time is only an indication and it is the customer’s responsibility to allow enough of a
guard band for their application. Prior to finalizing any software and while determining the maximum lock
time, take into account all device to device differences. Typically, applications set the maximum lock time
as an order of magnitude higher than the measured value. This is considered sufficient for all such device
to device variation.
Freescale recommends measuring the lock time of the application system by utilizing dedicated software,
running in FLASH, EEPROM, or RAM. This should toggle a port pin when the PLL is first configured
and switched on, then again when it goes from acquisition to lock mode and finally again when the PLL
lock bit is set. The resultant waveform can be captured on an oscilloscope and used to determine
the typical lock time for the microcontroller and the associated external application circuit. For example,
see Figure 4-7
t_Lock
t_ACQ
t_AL
INIT. LOW
SIGNAL ON PORT PIN
t_TRK COMPLETE AND LOCK SET
t_ACQ COMPLETE
PLL CONFIGURED AND SWITCHED ON
Figure 4-7. Typical Lock Time Waveform Example
NOTE
The filter capacitor should be fully discharged prior to making any
measurements.
MC68HC08AZ32A Data Sheet, Rev. 2
76
Freescale Semiconductor