Clock Generator Module (CGM)
For proper operation, the external filter capacitor must be chosen according to this
equation:
V
DDA
--------------
C
= C
FACT
F
f
RDV
For acceptable values of CFACT, see 4.8 Acquisition/Lock Time Specifications.
For the value of VDDA, choose the voltage potential at which the MCU is operating.
If the power supply is variable, choose a value near the middle of the range of
possible supply values.
This equation does not always yield a commonly available capacitor size, so round
to the nearest available size. If the value is between two different sizes, choose the
higher value for better stability. Choosing the lower size may seem attractive for
acquisition time improvement, but the PLL can become unstable. Also, always
choose a capacitor with a tight tolerance (±20 percent or better) and low
dissipation.
4.8.4 Reaction Time Calculation
The actual acquisition and lock times can be calculated using the equations here.
These equations yield nominal values under these conditions:
•
Correct selection of filter capacitor, CF, see 4.8.3 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. KACQ is the
K factor when the PLL is configured in acquisition mode, and KTRK is the K factor
when the PLL is configured in tracking mode. See 4.3.2.2 Acquisition and
Tracking Modes.
V
8
DDA
-------------- --------------
t
=
ACQ
f
K
RDV
ACQ
V
4
DDA
-------------- -------------
t
=
AL
f
K
RDV
TRK
t
= t
+ t
ACQ AL
Lock
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 4.3.2.3 Manual and Automatic
PLL Bandwidth Modes A certain number of clock cycles, nACQ, is required to
ascertain that the PLL is within the tracking mode entry tolerance, ∆TRK, before
Data Sheet
76
MC68HC908MR32 • MC68HC908MR16 — Rev. 6.0
Clock Generator Module (CGM)
MOTOROLA
FREESCALE [ Freescale ]
