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122001

FEATURES

§ Halts and restarts an out-of-control

microprocessor

§ Holds microprocessor in check during power

transients

§ Automatically restarts microprocessor after

power failure

§ Monitors pushbutton for external override

§ Accurate 5%, 10%, or 20% (3V only)

microprocessor power monitoring

§ Eliminates the need for discrete components

§ 20% tolerance for use with 3.0V systems

§ Pin- and function-compatible with the

MAX823/24/25 products

§ Low cost SOT-23-5 package

§ Industrial temperature range: -40°C to +85°C

DEVICE

RST

PBRST

DS181xA
DS181xB
DS181xC

X
X
X

X
X

x = "DS1814" for 5V devices
x = "DS1819" for 3V devices

PIN ASSIGNMENT

PIN DESCRIPTION

DS181xA
1

RST

Active Low Reset Output

GND

Ground

PBRST

Pushbutton Reset Input

Strobe Input

VCC

Power Supply

DS181xB
1

RST

Active Low Reset Output

GND

Ground

RST

Active High Reset Output

Strobe Input

VCC

Power Supply

DS181xC
1

RST

Active Low Reset Output

GND

Ground

RST

Active High Reset Output

PBRST

Pushbutton Reset Input

5 VCC

Power

Supply

DESCRIPTION

The DS1814 5-Volt MicroMonitor and the DS1819 3.3V MicroMonitor monitor three vital conditions for
a microprocessor: power supply, software execution, and external override. First, a precision temperature-
compensated reference and comparator circuit monitor the status of V

. When an out-of-tolerance

condition occurs, an internal power-fail signal is generated which forces the reset(s) to an active state.
When V

returns to an in-tolerance condition, the reset signal is kept in the active state for a minimum of

140ms to allow the power supply and processor to stabilize.

DS1814/DS1819

5V and 3.3V MicroMonitor

www.maxim-ic.com

SOT-23-5 PACKAGE

See Mech. Drawing

DS1814/DS1819

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The DS1814/DS1819 `A' and `C' perform pushbutton reset control. They debounce the pushbutton input
and generate an active reset pulse width of 140ms minimum. The DS1814/9 `A' and `B' versions perform
a watchdog function. The watchdog is an internal timer that forces the reset signals to the active state if
the strobe input does not change state every 1.12 seconds. The watchdog timer function can be disabled
by leaving the watchdog strobe input disconnected.

OPERATION
Power Monitor

The DS1814 detects out-of-tolerance power supply conditions and warns a processor-based system of
impending power failure. When V

falls below a preset level, a comparator outputs the signal

RST

(or

RST).

RST

(or RST) are excellent control signals for a microprocessor, as processing is stopped at the

last possible moment of valid V

. On power-up,

RST

(or RST) are kept active for a minimum of 140ms

to allow the power supply and processor to stabilize.

Pushbutton Reset

The DS1814 `A' and `C' provide an input pin for direct connection to a pushbutton reset (see Figure 2).
The pushbutton reset input requires an active low signal. Internally, this input is debounced and timed
such that a

RST

(or RST) signal of at least 140ms minimum will be generated. The 140ms delay

commences as the pushbutton reset input is released from the low level (see Figure 3).

Watchdog Timer

The DS1814/DS1819 `A' and `B' versions watchdog timer function forces

RST

(and RST) signal(s)

active when the

input is not clocked within the 1.12 second watchdog time-out period. Time-out of

the watchdog starts when

RST

(and RST) become(s) inactive. If a transition occurs on the

input pin

prior to time-out, the watchdog timer is reset and begins to time out again. If the watchdog timer is
allowed to time out, then the reset output(s) will go active for 140ms. The watchdog can be disabled by
floating (or tri-stating) the

input.

The

input can be derived from many microprocessor outputs. The most typical signals used are the

microprocessor data I/O signals and control signals. When the microprocessor functions normally, these
signals would as a matter of routine cause the watchdog to be reset prior to time-out. To guarantee that
the watchdog timer does not time out, a transition must occur at or less than 1.12 seconds. A typical
circuit example is shown in Figure 4. The DS1814/19 watchdog function can be disabled at any time by
tri-starting the strobe input. A sample circuit to disable the watchdog is shown in Figure 6.

DS1814/DS1819

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TIMING DIAGRAM: POWER-UP Figure 8

RST

VALID TO 0 VOLTS V

Figure 9

OUTPUT VALID CONDITIONS

The

RST

output uses a push-pull output which can maintain a valid output down to 1.0V V

. To sink

current below 1.0V a resistor can be connected form

RST

to Ground (see Figure 9.) This arrangement

will maintain a valid value on

RST

during both power-up and -down but will draw current when

RST

in the high state. A value of about 100k

W should be adequate in most situations. The output with a

resistor pull-down can maintain a valid reset down to V

equal to 0V.

DS1814/DS1819

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ABSOLUTE MAXIMUM RATINGS*

Voltage on V

Pin Relative to Ground

-0.5V to +7.0V

Voltage on I/O Relative to Ground

-0.5V to V

+ 0.5V

Operating Temperature Range

-40

°C to +85°C

Storage Temperature Range

-55

°C to +125°C

Soldering Temperature

260

°C for 10 seconds

* This is a stress rating only and functional operation of the device at these or any other conditions

above those indicated in the operation sections of this specification is not implied. Exposure to
absolute maximum rating conditions for extended periods of time may affect reliability.

RECOMMENDED DC OPERATING CONDITIONS

(-40

C to +85

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS NOTES

Supply Voltage

1.0

5.5

and

PBRST

Input High Level

0.8 V

and

PBRST

Input Low Level

0.3 V

DC ELECTRICAL CHARACTERISTICS (-40

°C to +85°C; V

= 1.2V to 5.5V)

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS NOTES

Trip Point DS1814-5

CCTP

4.50

4.75

Trip Point DS1814-10

CCTP

4.25

4.50

Trip Point DS1819-5

CCTP

3.00

3.15

Trip Point DS1819-10

CCTP

2.85

3.00

Trip Point DS1819-20

CCTP

2.55

2.70

Reset Trip Point Hysteresis (DS1814)

Reset Trip Point Hysteresis (DS1819)

Output Current @ 2.4V

350

Output Current @ 0.4V

3.2

Output Voltage @ -30

*.8

*.9

Operating Current DS1814

Operating Current DS1819

Input Current

= V

, Time Averaged

120

160

Input Current

= GND, Time Averaged

-20

-15

DS1814/DS1819

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AC ELECTRICAL CHARACTERISTICS (-40

°C to +85°C; V

= 1.2V to 5.5V)

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS NOTES

PBRST

= V

Reset Active Time

RST

140

200

280

Pulse Width

Watchdog Time-out

1.12

1.60

2.40

Detect to RST and

RST

RPD

Slew Rate (rising edge)

12.5

Detect to RST and

RST

RPU

140

200

280

PBRST

Stable Low to RST and

RST

PDLY

500

CAPACITANCE

= 25

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS NOTES

Input Capacitance

Output Capacitance

OUT

NOTES:

1) All voltages are referenced to ground.

PBRST

is internally pulled up to V

with an internal impedance of 50k

W typical.

3) Measured with V

³ 2.7V.

4) Measured with outputs open, V

£ 5.5V, and all inputs at V

or Ground, except the

input at

<10kHz duty cycle-square wave or

input open.

5) Measured with outputs open, V

£ 3.6V, and all inputs at V

or Ground, except the

input at

<10kHz duty cycle-square wave or

input open.

6) The

input current is defined as the average input current when

is driven high or low. This input

must be driven high or low by an output that can source or sink at least 200

mA when active.

7) Noise immunity: pulses <100ns at pushbutton input will not cause a reset.
8) Must not exceed t

minimum.

9) Noise immunity: pulses <2

ms at V

CCTP

minimum will not cause a reset and V

CCTP

- V

= 100mV.

Part Number DS1819