Clock Generator Module (CGM)
MC68HC08KH12A Data Sheet, Rev. 1.1
78
Freescale Semiconductor
8.8.4 Reaction Time Calculation
The actual acquisition and lock times can be calculated using the equations below. These equations yield
nominal values under the following conditions:
Correct selection of filter capacitor, C
F
(See
8.8.3 Choosing a Filter Capacitor
.)
Room temperature operation
Negligible external leakage on CGMXFC
Negligible noise
The K factor in the equations is derived from internal PLL parameters. K
ACQ
is the K factor when the PLL
is configured in acquisition mode, and K
TRK
is the K factor when the PLL is configured in tracking mode.
(See
8.3.4 Acquisition and Tracking Modes
.) Reaction time is based on an initial frequency error, (f
DES
–
f
ORIG
)/f
DES
, of not more than
±
100 percent.
NOTE
The inverse proportionality between the lock time and the reference
frequency.
In automatic bandwidth control mode, the acquisition and lock times are quantized into units based on the
reference frequency. (See
8.3.5 Manual and Automatic PLL Bandwidth Modes
.) A certain number of clock
cycles, n
ACQ
, is required to ascertain that the PLL is within the tracking mode entry tolerance,
TRK
, before
exiting acquisition mode. A certain number of clock cycles, n
TRK
, is required to ascertain that the PLL is
within the lock mode entry tolerance,
LOCK
. Therefore, the acquisition time, t
ACQ
, is an integer multiple of
n
ACQ
/f
RDV
, and the acquisition to lock time, t
AL
, is an integer multiple of n
TRK
/f
RDV
.
In manual mode, it is usually necessary to wait considerably longer than t
LOCKMAX
before selecting the PLL
clock (See
8.3.8 Base Clock Selector Circuit
.), because the factors described in
8.8.2 Parametric
Influences on Reaction Time
may slow the lock time considerably. Automatic bandwidth mode is
recommended for most users.
tACQ
fRDV
VDDA
KACQ
-------8
=
tAL
fRDV
tACQ
VDDA
KTRK
tAL
-----4
=
tLOCKMAX
256tVRDV
+
+
=