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TC1784
System Control Unit (SCU)
User´s Manual
3-22
V1.1, 2011-05
32-bit SCU, V1.18
•
PLLERAYSTAT.VCOBYST = 0
•
PLLERAYSTAT.VCOLOCK = 1
•
OSCCON.PLLLV = 1
•
OSCCON.PLLHV = 1
Operation on the Normal 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.
The system operation frequency is controlled in the Normal Mode by the values of the
two dividers: N and K2. A modification of the divider N has a direct influence to the VCO
frequency and lead to a loss of the VCO Lock status. A modification of the K2-divider has
no impact on the VCO Lock status but still changes the PLL_ERAY output frequency.
When the frequency of the Normal Mode should be modified or entered the following
sequence should be followed:
First the Prescaler Mode should be configured and entered. For more details see the
Prescaler Mode.
The NMI trap generation for the VCO Lock should be disabled.
While the Prescaler Mode is used the Normal Mode can be configured and checked for
a positive VCO Lock status. The first target frequency of the Normal Mode should be
selected in a way that it matches or is only slightly higher as the one used in the
Prescaler Mode. This avoids big changes in the system operation frequency and
therefore power consumption when switching later from Prescaler Mode to Normal
Mode. The N divider should be selected in the following way:
•
Selecting N in a way that
f
VCO
is in the lower area of its allowed values leads to a
slightly reduced power consumption but to a slightly increased jitter
•
Selecting N in a way that
f
VCO
is in the upper area of its allowed values leads to a
slightly increased power consumption but to a slightly reduced jitter
After the N and K2 dividers are updated for the first configuration the indication of the
VCO Lock status should be await (PLLERAYSTAT.VCOLOCK = 1).
Note: It is recommended to reset the VCO Lock detection (PLL
ERAY
CON0.RESLD = 1)
after the new values of the dividers are configured to get a defined VCO lock check
time.
When this happens the switch from Prescaler Mode to Normal Mode can be done.
Normal Mode is requested by clearing PLLERAYCON.VCOBYP. The Normal Mode is
entered when the status bit PLLERAYSTAT.VCOBYST is cleared.
Now the Normal Mode is entered. The NMI status flag for the VCO Lock trap should be
cleared and then enabled again. The intended PLL_ERAY output target frequency can
not be configured by changing only the K2-Divider.
Содержание TC1784
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