MIC708TMY Datasheet, PDF (8/10 Pages) Micrel Semiconductor – μP Supervisory Circuit

Micrel, Inc.
Application Information
An additional comparator which is independent of the
other functions on the MIC706P and MIC708 is provided
for early warning of power failure. An external voltage
divider 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 V_CC 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 shutdown procedures prior to
power loss. Hysteresis can be added to this comparator
with external resistors, as is commonly done with any
comparator.
MIC706P/R/S/, MIC708R/S/T
Watchdog Timer
The microprocessor can be monitored by connecting the
WDI pin (watchdog input) to a bus line or an I/O line. If a
transition doesn't occur on the WDI pin within in the
watchdog timeout period, then /WDO will go low. A
minimum pulse of 100ns or any transition low-to-high or
high-to-low on 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 period.
If VCC falls below the reset threshold voltage, then /WDO
goes low immediately regardless of WDI. /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
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.
Figure 2. Power-Fail Comparator
May 28, 2014
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Revision 2.0