MAX6830 データシートの表示（PDF） - Maxim Integrated

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MAX6830

Dual Ultra-Low-Voltage SOT23 µP Supervisors with Manual Reset and Watchdog Timer

Maxim Integrated 

MAX6826-MAX6831

Dual Ultra-Low-Voltage SOT23 µP Supervisors with

Manual Reset and Watchdog Timer

VCC

tRST

RESET*

tRP

tWD

tRP

WDI

*RESET IS THE INVERSE OF RESET.

Figure 4. Watchdog Timing Relationship

VMONITOR

MAX6826

MAX6827

R1

MAX6828

RSTIN

R2

VCC ≥ VRSTIN

Figure 5. Monitoring a Voltage

outputs. Connect a normally open momentary switch

from MR to GND to create a manual-reset function;

external debounce circuitry is not required. If MR is dri-

ven from long cables or the device is used in a noisy

environment, connect a 0.1µF capacitor from MR to

GND to provide additional noise immunity.

Watchdog Input

The watchdog circuit monitors the µP’s activity. If the

µP does not toggle (low to high or high to low) the

watchdog input (WDI) within the watchdog timeout

period (1.6s nominal), reset asserts for the reset timeout

period. The internal 1.6s timer can be cleared by either

a reset pulse or by toggling WDI. The WDI can detect

pulses as short as 50ns. While reset is asserted, the

timer remains cleared and does not count. As soon as

reset is released, the timer starts counting (Figure 4).

Disable the watchdog function by leaving WDI uncon-

nected or by three-stating the driver connected to WDI.

The watchdog input is internally driven low during the

first 7/8 of the watchdog timeout period and high for the

last 1/8 of the watchdog timeout period. When WDI is left

unconnected, this internal driver clears the 1.6s timer

every 1.4s. When WDI is three-stated or unconnected,

the maximum allowable leakage current is 10µA and the

maximum allowable load capacitance is 200pF.

VCC

MAX6828

MAX6831 RESET

RESET

GENERATOR

GND

VCC

RESET

µP

GND

Figure 6. Interfacing Open-Drain RESET to µPs with

Bidirectional Reset I/O

Adjustable Reset Thresholds

The MAX6826/MAX6827/MAX6828 provide a user

adjustable input to monitor a second voltage. The

threshold voltage at RSTIN is typically 0.63V. To moni-

tor a voltage higher than 0.63V, connect a resistor-

divider to the circuit as shown in Figure 5. The

threshold at VMONITOR is:

VMONITOR_TRIP

=

0.63V

R1+ R2

 R2 

Note that RSTIN is powered by VCC, and its voltage must

therefore remain lower than VCC.

Applications Information

Watchdog Input Current

The WDI inputs are internally driven through a buffer and

series resistor from the watchdog timer (Figure 1). When

WDI is left unconnected, the watchdog timer is serviced

within the watchdog timeout period by a 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 watch-dog timeout

period, pulsing it low-high-low once within the first 7/8 of

the watchdog timeout period to reset the watchdog timer.

If WDI is externally driven high for the majority of the

timeout period, up to 160µA can flow into WDI.

Interfacing to µPs with Bidirectional

Reset Pins

Since the RESET output on the MAX6828/MAX6831

is open drain, these devices interface easily with µPs

that have bidirectional reset pins, such as the Motorola

68HC11. Connecting the µP supervisor’s RESET output

directly to the microcontroller’s RESET pin with a single

pullup resistor allows the device to assert a reset

(Figure 6).

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