6

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

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.

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

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

+

-

1.25V

R1

R2

Unregulated DC

PFO

PFI

Figure 3. Power Fail Comparator

Circuit Description

Figure 4. Reset Timing Diagram

VCC

MR

RESET

WDO

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

µP Supervisory Circuits