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USER’S GUIDE
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62
The original contents of those Special Function regis-
ters that are initialized by a reset are lost.
Voltage Monitoring Circuitry
The on–chip voltage monitoring circuitry automatically
places the microprocessor in its Data Retention state in
the absence of V
CC
. It insures that the proper internal
control signals are generated and that power from the
lithium cell is applied at the proper times so that the Pro-
gram/Data RAM, data in the Scratchpad Registers and
certain Special Function Registers remain unchanged
when V
CC
is cycled on and off. In addition, an interrupt is
available for signaling the processor of an impending
power fail condition so that the operational state of the
processor can be saved just prior to entering the Data
Retention.
The voltage monitoring circuitry recognizes three volt-
age thresholds below nominal operating voltage. These
thresholds are identified as V
PFW
(Power Fail Warning
voltage), V
CCmin
(minimum operating voltage), and V
LI
(lithium supply) voltage. These thresholds are used to
initiate required actions within the microprocessor dur-
ing situations when V
CC
power is cycled on and off. The
timing diagram shown in Figure 7–2 illustrates key inter-
nal activities during power cycling.
SECURE MICROCONTROLLER POWER CYCLING TIMING Figure 7–2
t
CSU
t
POR
t
F
V
PFW
V
CCMIN
V
LI
V
CC
INTERRUPT
SERVICE
ROUTINE
CLOCK
OSC.
INTERNAL
RESET
LITHIUM
CURRENT