DS1814/DS1819
5V and 3.3V MicroMonitor
www.maxim-ic.com
FEATURES PIN ASSIGNMENT
Halts and restarts an out-of-control 32 1
microprocessor
Holds microprocessor in check during power
transients
Automatically restarts microprocessor after
power failure
45
Monitors pushbutton for external override
SOT-23-5 PACKAGE
Accurate 5%, 10%, or 20% (3V only)
See Mech. Drawing
microprocessor power monitoring
Eliminates the need for discrete components
20% tolerance for use with 3.0V systems
PIN DESCRIPTION
Pin- and function-compatible with the
DS181xA
MAX823/24/25 products
1 RST Active Low Reset Output
Low cost SOT-23-5 package
2 GND Ground
Industrial temperature range: -40C to +85C
3 PBRST Pushbutton Reset Input
4 ST Strobe Input
DEVICE RST WD
RST PBRST
5 VCC Power Supply
DS181xA X X X
DS181xB
DS181xB X X X
1 RST Active Low Reset Output
DS181xC X X X
2 GND Ground
x =DS1814" for 5V devices
3 RST Active High Reset Output
x =DS1819" for 3V devices
4 ST Strobe Input
5 VCC Power Supply
DS181xC
1 RST Active Low Reset Output
2 GND Ground
3 RST Active High Reset Output
4 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
CC
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
CC
140ms to allow the power supply and processor to stabilize.
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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
CC
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
CC
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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