TC1784
System Control Unit (SCU)
User´s Manual
3-12
V1.1, 2011-05
32-bit SCU, V1.18
•
PLLSTAT.VCOBYST = 1
•
OSCCON.PLLLV = 1
Operation on the Prescaler Mode does require an input clock frequency of
f
OSC
.
Therefore it is recommended to check and monitor if an input frequency
f
OSC
is available
at all by checking OSCCON.PLLLV. For a better monitoring also the upper frequency
can be monitored via OSCCON.PLLHV.
For the Prescaler Mode there are no requirements regarding the frequency of
f
OSC
.
The system operation frequency is controlled in the Prescaler Mode by the value of the
K1-Divider. When the value of PLLCON1.K1DIV was changed the next update of this
value should not be done before bit PLLSTAT.K1RDY is set.
Note: Changing the system operation frequency by changing the value of the K1-Divider
has a direct coupling to the power consumption of the device. Therefore this has
to be done carefully.
Depending on the selected divider value of the K1-Divider the duty cycle of the clock is
selected. This can have an impact for the operation with an external communication
interface. The duty cycles values for the different K1-divider values are defined in the
Data Sheet.
The Prescaler Mode is requested from the Freerunning or Normal Mode by setting bit
PLLCON.VCOBYP. The Prescaler Mode is entered when the status bit
PLLSTAT.VCOBYST is set. Before the Prescaler Mode is requested the K1-Divider
should be configured with a value generating a PLL output frequency
f
PLL
that matches
the one generated by the Freerunning or Normal Mode as much as possible. In this way
the frequency change resulting out of the mode change is reduced to a minimum.
The Prescaler Mode is requested to be left by clearing bit PLLCON.VCOBYP. The
Prescaler Mode is left when the status bit PLLSTAT.VCOBYST is cleared.
Configuration and Operation of the Normal Mode
In Normal Mode, the PLL is running at the external frequency
f
OSC
and
f
PLL
is divided
down by a factor P, multiplied by a factor N and then divided down by a factor K2.
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