Freescale Semiconductor MC68HC08KH12, Datasheet

Page: 95 / 262

MC68HC(7)08KH12
—
Rev. 1.1 Advance Information
Freescale Semiconductor
95
2. Choose a practical PLL (crystal) reference frequency, f
RCLK
, and
the reference clock divider, R.
Frequency errors to the PLL are corrected at a rate of f
RCLK
/R. For
stability and lock time reduction, this rate must be as fast as
possible. The VCO frequency must be an integer multiple of this
rate. The relationship between the VCO frequency f
VCLK
and the
reference frequency f
RCLK
is
Choose the reference divider R = 1 for fast lock. Choose a f
RCLK
frequency with an integer divisor of f
BUS
and solve for N.
3. Program the PLL registers accordingly:
a.
In the PRE bits of the PLL control register (PCTL), program
the binary equivalent of P.
b.
In the PLL multiplier select register low (PMSL) and the PLL
multiplier select register high (PMSH), program the binary
equivalent of N.
c.
In the PLL reference divider select register (PRDS), program
the binary coded equivalent of R.
Table 8-1 provides a numeric example (numbers are in hexadecimal
notation):
8.4.7 Special Programming Exceptions
The programming method described in 8.4.6 Programming the PLL
does not account for three possible exceptions. A value of zero for R, N,
or L is meaningless when used in the equations given. To account for
these exceptions:
Table 8-1. CGM Numeric Example
f
BUS
f
RCLK
P N R
6MHz 6MHz 1 004 1
f
VCLK
2
P
N
×
R
-----------------
f
RCLK
( )
=
hence: 48MHz
2
P
N
×
R
-----------------
f
RCLK
( )
=