Micrel, Inc.

MIC706P/R/S/, MIC708R/S/T

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

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 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.

May 28, 2014

8

Revision 2.0