DS1814/DS1819
5V and 3.3V MicroMonitor
www.maxim-ic.com
FEATURES
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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
RST
PIN ASSIGNMENT
3
2
1
4
5
SOT-23-5 PACKAGE
See Mech. Drawing
PIN DESCRIPTION
DS181xA
1
RST
2 GND
PBRST
3
4
ST
5 VCC
DS181xB
1
RST
2 GND
3 RST
4
ST
5 VCC
DS181xC
1
RST
2 GND
3 RST
4
PBRST
5 VCC
Active Low Reset Output
Ground
Pushbutton Reset Input
Strobe Input
Power Supply
DEVICE
RST
PBRST
WD
X
X
DS181xA
X
DS181xB
X
X
DS181xC
X
X
x = "DS1814" for 5V devices
x = "DS1819" for 3V devices
X
X
Active Low Reset Output
Ground
Active High Reset Output
Strobe Input
Power Supply
Active Low Reset Output
Ground
Active High Reset Output
Pushbutton Reset Input
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
CC
. 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
CC
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.
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122001
DS1814/DS1819
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
CC
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
CC
. 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
ST
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
ST
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
ST
input.
The
ST
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.
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DS1814/DS1819
DS1814/DS1819 BLOCK DIAGRAM
Figure 1
*Notes:
ST
available on'A' and 'B' versions
PBRST
available on 'A' and 'C' versions
RST available on 'B' and 'C' versions
PUSHBUTTON RESET
Figure 2
TIMING DIAGRAM: PUSHBUTTON RESET
Figure 3
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DS1814/DS1819
WATCHDOG TIMER
Figure 4
TIMING DIAGRAM: STROBE INPUT
Figure 5
WATCHDOG DISABLE CIRCUIT
Figure 6
TIMING DIAGRAM: POWER-DOWN
Figure 7
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DS1814/DS1819
TIMING DIAGRAM: POWER-UP
Figure 8
RST
VALID TO 0 VOLTS V
CC
Figure 9
OUTPUT VALID CONDITIONS
The
RST
output uses a push-pull output which can maintain a valid output down to 1.0V V
CC
. 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
is
in the high state. A value of about 100kW should be adequate in most situations. The output with a
resistor pull-down can maintain a valid reset down to V
CC
equal to 0V.
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