MIC706P Datasheet, PDF (6/8 Pages) Micrel Semiconductor – μP Supervisory Circuits

MIC706P/R/S/T, MIC708R/S/T µP Supervisory Circuits
Circuit Description
Power Fail Warning
An additional comparator which is independent of other
functions on the MIC706/MIC708 is provided for early
warning of power failure. An external voltage divider
Unregulated DC
R1
PFI
R2
+
1.25V
-
PFO
the WDI pin will reset the watchdog timer. The output of
the watchdog timer (WDO) will remain high if WDI sees
a valid transition within the watchdog timeout period. If
VCC falls below the reset threshold voltage then WDO
goes low immediately regardless of WDI. Likewise,
WDO goes high immediately after VCC exceeds the
reset threshold. WDO can be connected to MR to
generate a reset pulse upon watchdog timeout.
Microprocessor Reset
The RESET pin is asserted whenever VCC falls below
the reset threshold voltage or when MR goes low. The
reset pin remains asserted for a period of 200ms after
Figure 3. Power Fail Comparator
can be used to compare unregulated DC to an internal
1.25V reference. The voltage divider ratio on the input
of the power fail comparator (PFI) can be chosen so as
to trip the power fail comparator a few milliseconds
before VCC _{falls below the maximum reset threshold}
voltage. The output of the power fail comparator (PFO)
can be used to interrupt the microprocessor when used
in this mode and execute shut-down procedures prior to
power loss.
Watchdog Timer
The microprocessor can be monitored by connecting
the WDI pin (watchdog input) to a bus line or I/O line. If
a transition doesn’t occur on the WDI pin within the
watchdog timeout period, then WDO will go low. A
minimum pulse of 50ns (4.5V supply) or 100ns (2.7V
supply) or any transition low-to-high or high-to-low on
VCC
tMR
MR
tMD
tRS
RESET
WDO
VRT
tRS
Figure 4. Reset Timing Diagram
VCC has risen above the reset threshold voltage and
MR goes high. The reset function ensures the
microprocessor is properly reset and powers up into a
known condition after a power failure. RESET will
remain valid with VCC as low as 1.4V.
6