Micrel, Inc.

MIC705/706/707/708

Application Information

Microprocessor Reset

reset threshold voltage or when /MR goes low. The reset

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

as 1.4V.

Power-Fail Warning

An additional comparator which is independent of the

other functions on the MIC706/706/707/708 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

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.

Figure 2. Power-Fail Comparator

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 the

watchdog timeout period, then /WDO will go low. A

minimum pulse of 50ns 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 left floating. If VCC falls below the reset threshold

voltage, then /WDO goes low immediately regardless of

WDI.

September 15, 2014

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Revision 2.1