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TC1784
System Control Unit (SCU)
User´s Manual
3-23
V1.1, 2011-05
32-bit SCU, V1.18
Depending on the selected divider value of the K2-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 K2-divider values are defined in the
Data Sheet. This can result in multiple changes of the K2-Divider to avoid to big
frequency changes. Between the update of two K2-Divider values 6 cycles of
f
PLL_ERAY
should be waited.
PLL_ERAY VCO Lock Detection
The PLL_ERAY has a lock detection that supervises the VCO part of the PLL_ERAY in
order to differentiate between stable and instable VCO circuit behavior. The lock detector
marks the VCO circuit and therefore the output
f
VCO
of the VCO as instable if the two
inputs
f
REF
and
f
DIV
differ too much. Changes in one or both input frequencies below a
level are not marked by a loss of lock because the VCO can handle such small changes
without any problem for the system.
PLL_ERAY VCO Loss-of-Lock Event
The PLL_ERAY may become unlocked, caused by a break of the crystal or the external
clock line. In such a case, an NMI trap is generated if the according NMI trap is enabled.
Additionally, the OSC clock input
f
OSC
is disconnected from the PLL_ERAY VCO to avoid
unstable operation due to noise or sporadic clock pulses coming from the oscillator
circuit. Without a clock input
f
OSC
, the PLL_ERAY gradually slows down to its VCO base
frequency and remains there. This automatic feature can be disabled by setting bit
PLLERAYCON0.OSCDISCDIS. If this bit is cleared the OSC clock remains connected
to the VCO.
VCO Power Down
Mode
The PLL_ERAY offers a VCO Power Down Mode. This mode can be entered to save
power within the PLL_ERAY. The VCO Power Down Mode is entered by setting bit
PLLCON0.VCOPWD. While the PLL_ERAY is in VCO Power Down Mode only the
Prescaler Mode is operable. Please note that selecting the VCO Power Down Mode
does not automatically switch to the Prescaler Mode. So before the VCO Power Down
Mode is enter the Prescaler Mode must be active.
PLL_ERAY Power Down Mode
The PLL_ERAY offers a Power Down Mode. This mode can be entered to save power if
the PLL_ERAY is not needed at all. The Power Down Mode is entered by setting bit
PLLCON0.PLLPWD. While the PLL_ERAY is in Power Down Mode no PLL_ERAY
output frequency is generated.
3.1.1.5
Clock Control Unit
The Clock Control Unit (CCU) receives the clock that is created by the two PLLs
f
PLL
and
f
PLL_ERAY
.
Содержание TC1784
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