mc68hc908mr32 Freescale Semiconductor, Inc, mc68hc908mr32 Datasheet - Page 62

• Manufacturer Part Number: mc68hc908mr32
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Clock Generator Module (CGM)
Exceeding the recommended maximum bus frequency or VCO frequency can crash the MCU.
4.3.2.5 Special Programming Exceptions
The programming method described in
exceptions. A value of 0 for N or L is meaningless when used in the equations given. To account for these
exceptions:
4.3.3 Base Clock Selector Circuit
This circuit is used to select either the crystal clock, CGMXCLK, or the VCO clock, CGMVCLK, as the
source of the base clock, CGMOUT. The two input clocks go through a transition control circuit that waits
up to three CGMXCLK cycles and three CGMVCLK cycles to change from one clock source to the other.
During this time, CGMOUT is held in stasis. The output of the transition control circuit is then divided by
62
5. Calculate the bus frequency, f
6. If the calculated f
7. Using the value 4.9152 MHz for f
8. Calculate the VCO center-of-range frequency, f
9. Program the PLL registers accordingly:
•
•
another f
multiplier controls the frequency range of the PLL.
between the minimum and maximum frequencies attainable by the PLL.
For proper operation,
A 0 value for N is interpreted exactly the same as a value of 1.
A 0 value for L disables the PLL and prevents its selection as the source for the base clock. See
4.3.3 Base Clock Selector
a. In the upper four bits of the PLL programming register (PPG), program the binary equivalent
b. In the lower four bits of the PLL programming register (PPG), program the binary equivalent
Example: L =
Example: f
of N.
of L.
RCLK
.
VRS
Example: N =
BUS
f
4.9152 MHz
VRS
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
= 7 x 4.9152 MHz = 34.4 MHz
32 MHz
is not within the tolerance limits of the application, select another f
L = round
– f
VCLK
Circuit.
BUS
32 MHz
4 MHz
| ≤
NOM
= 7 MHz
(
4.3.2.4 Programming the PLL
, and compare f
f
f
f
NOM
VCLK
NOM
, calculate the VCO linear range multiplier, L. The linear range
2
f
VRS
f
BUS
CAUTION
= 8 MHz
)
= L x f
=
f
VRS
VCLK
4
NOM
BUS
. The center-or-range frequency is the midpoint
with f
BUSDES
does not account for possible
.
Freescale Semiconductor
BUSDES
or