MAX791M(1995) 查看數據表（PDF） - Maxim Integrated

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MAX791M
(Rev.:1995)

Microprocessor Supervisory Circuit

Maxim Integrated 

Microprocessor Supervisory Circuit

15

RESET

MAX791

TO µP RESET

10k

WDI

WDPO

WDO

1.6sec

100ns MIN

70ns

–F—i—g—u—re– 4. Adding an external pull-down resistor ensures

RESET is valid with VCC down to GND.

–——– –———–

Figure 5. WDI, WDO and WDPO Timing Diagram (VCC mode)

RESET Output

The MAX791’s RESET output ensures that the µP pow-

ers up in a known state, and prevents code-execution

errors during power-down or brownout conditions.

The RESET output is active low, and typically sinks

3.2mA at 0.1V saturation voltage in its active state.

When deasserted, RESET sources 1.6mA at typically

VOUT - 0.5V. When no backup battery is used, RESET

output is guaranteed to be valid down to VCC = 1V, and

an external 10kΩ pull-down resistor on RESET ensures

that RESET will be valid with VCC down to GND (Figure

4). As VCC goes below 1V, the gate drive to the RESET

output switch reduces accordingly, increasing the

rDS(ON) and the saturation voltage. The 10kΩ pull-down

resistor ensures the parallel combination of switch plus

resistor is around 10kΩ and the output saturation volt-

age is below 0.4V while sinking 40µA. When using a

10kΩ external pull-down resistor, the high state for the

RESET output with VCC = 4.75V is 4.5V typical. For bat-

tery voltages ≥ 2V connected to VBATT, RESET

remains valid for VCC from 0V to 5.5V.

RESET will be asserted during the following conditions:

1) VCC < 4.65V (typ)

2) MR < 1.25V (typ)

3) RESET remains asserted for 200ms (typ) after VCC

rises above 4.65V or after MR has exceeded

1.25V.

The MAX791 battery-switchover comparator does not

affect RESET assertion. However, RESET is asserted in

battery-backup mode since VCC must be below the

reset threshold to enter this mode.

Watchdog Function

The watchdog monitors µP activity via the Watchdog

Input (WDI). If the µP becomes inactive, WDO and

WDPO are asserted. To use the watchdog function,

connect WDI to a bus line or µP I/O line. If WDI remains

high or low for longer than the watchdog timeout period

(1.6sec nominal), WDPO and WDO are asserted, indi-

cating a software fault condition (see Watchdog Output

and Watchdog-Pulse Output sections).

Watchdog Input

A change of state (high to low, low to high, or a mini-

mum 100ns pulse) at WDI during the watchdog period

resets the watchdog timer. The watchdog default time-

out is 1.6sec. Select alternative timeout periods by con-

necting an external capacitor from SWT to GND (see

Selecting an Alternative Watchdog Timeout section).

To disable the watchdog function, leave WDI floating.

An internal resistor network (100kΩ equivalent imped-

ance at WDI) biases WDI to approximately 1.6V.

Internal comparators detect this level and disable the

watchdog timer. When VCC is below the reset thresh-

old, the watchdog function is disabled and WDI is dis-

connected from its internal resistor network, thus

becoming high impedance.

Watchdog Output

WDO remains high if there is a transition or pulse at

WDI during the watchdog-timeout period. The watch-

dog function is disabled and WDO is a logic high when

VCC is below the reset threshold, battery-backup mode

is enabled, or WDI is an open circuit. In watchdog

mode, if no transition occurs at WDI during the watch-

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