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USER’S GUIDE
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64
threshold, the Power On Reset cycle will be executed as
before. As a result, no special processing is required in
software to accommodate this case.
In the case that V
CC
dips without going below V
LI
, the
PFW flag will be set and a Power Fail Warning interrupt
will still occur when V
CC
drops below the V
PFW
thresh-
old. The PFW flag will remain set until it is cleared by ei-
ther a reset of the flag by the software or by a Power On
cycle. If it is cleared while V
CC
is still below the V
PFW
threshold, it will be immediately set again. If it is cleared
after V
CC
has risen back above the V
PFW
threshold,
then it will remain cleared until the next time V
CC
goes
below V
PFW
.
As long as the PFW flag is set, an interrupt condition is
defined if EPFW is set. If the software executes a ser-
vice routine in response to a PFW interrupt and exits the
service routine with the PFW flag still set, then the pro-
cessor will be immediately interrupted again. In a typical
application, however, the Power Fail Interrupt service
routine would test the PFW flag in a conditional loop to
determine if V
CC
has risen back above V
PFW
and would
then return control to the main program in response to
the event. See Figure 7–3 for details.
SECURE MICROCONTROLLER POWER MANAGEMENT Figure 7–3
POR = 0?
RUN NORMAL
RESET ROUTINE
SET POR
PFW = 1?
LONG POWER–UP
SLEW RATE, KEEP
CHECKING
SET EPFW
POWER–FAIL
INTERRUPT
POWER ON
BOOT ROUTINE
NO
NO
YES
YES
POWER–FAIL WARNING
INTERRUPT VECTOR
POWER HAS FALLEN BELOW VPFW,
CLEAR PFW FLAG BY READING
(READ PCON)
SAVE CRITICAL DATA TO NVRAM;
PREPARE FOR POWER FAILURE
POWER HAS
RETURNED,
RESUME
VOLTAGE IS STILL BELOW
VPFW, NOT SAFE TO RUN
NO
YES
OPERATION
PFW = 1?
RESET VECTOR