Interfacing to µPs with

Bidirectional Reset Pins

Since

RESET is open-drain, the MAX6301 interfaces

easily with µPs that have bidirectional reset pins, such

as the Motorola 68HC11 (Figure 7). Connecting

RESET

directly to the µP’s reset pin with a single pull-up allows

either device to assert reset.

In addition to issuing a reset to the µP during power-up,

power-down, and brownout conditions, these supervisors

are relatively immune to short-duration negative-going

transients (glitches). The graph Maximum Transient

Duration vs. Reset Threshold Overdrive in the

Typical

Operating Characteristics shows this relationship.

The area below the curves of the graph is the region in

which these devices typically do not generate a reset

pulse. This graph was generated using a negative-

nitude indicated (reset-threshold overdrive). As the

magnitude of the transient increases (farther below the

reset threshold), the maximum allowable pulse width

below the reset threshold and lasts 50µs or less will not

cause a reset pulse to be issued.

Watchdog Input Current

Extended Mode

nally driven through a buffer and series resistor from

the watchdog counter (Figure 8). When WDI is left

unconnected, the watchdog timer is serviced within the

watchdog timeout period by a very brief low-high-low

pulse from the counter chain. For minimum watchdog

input current (minimum overall power consumption),

leave WDI low for the majority of the watchdog timeout

period, pulsing it low-high-low (>30ns) once within the

period to reset the watchdog timer. If instead WDI is

externally driven high for the majority of the timeout

period, typically 70µA can flow into WDI.

Normal Mode

In normal mode (WDS = GND), the internal buffer that

drives WDI is disabled. In this mode, WDI is a standard

CMOS input and leakage current is typically 100pA,

regardless of whether WDI is high or low.

+5V, Low-Power µP Supervisory Circuits

with Adjustable Reset/Watchdog

_______________________________________________________________________________________

9

0.1µF

MAX6301

MAX6302

MAX6303

MAX6304

RESET IN

R2

R1

Figure 6. Adding a Manual Reset Function

MAX6301

RESET

RESET

GND

µP

RESET TO

OTHER SYSTEM

COMPONENTS

0.1µF

4.7k

Figure 7. Interfacing to µPs with Bidirectional Reset I/O Pins

MAX6301

MAX6302

MAX6303

MAX6304

WATCHDOG

TIMER

TO RESET

GENERATOR

TO MODE

CONTROL

WDI

WDS

Figure 8. Watchdog Input Structure