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DS1707/DS1708
3.3 and 5.0-Volt MicroMonitor
www.maxim-ic.com
FEATURES
§
§
§
§
§
§
§
§
§
Holds microprocessor in check during power
transients
Automatically restarts microprocessor after
power failure
Monitors pushbutton for external override
Accurate 5%, 10% or 20% resets for 3.3V
systems and 5% or 10% resets for 5.0V
systems
Eliminates the need for discrete components
20% tolerance compatible with 3.0V systems
Pin compatible with the MAXIM
MAX707/MAX708 in 8-pin DIP, 8-pin SOIC
packages
8-pin DIP, 8-pin and
m-SOP
SOIC and 8-pin
m-SOP
packages available
Industrial temperature range -40°C to +85°C
PIN ASSIGNMENT
PBRST
V
CC
GND
IN
1
2
3
4
8
7
6
5
RST
RST
NC
NMI
8-Pin DIP (300-mil)
PBRST
V
CC
GND
IN
1
2
3
4
8
7
6
5
RST
RST
NC
NMI
8-Pin SOIC (150-mil)
RST
RST
PBRST
V
CC
1
2
3
4
8
7
6
5
NC
NMI
IN
GND
8-Pin
m-SOP
(118-mil)
See Mech. Drawings Section on website
DS1707 and DS1708_R/S/T
PIN DESCRIPTION
PBRST
V
CC
GND
IN
NMI
NC
RST
RST
- Pushbutton Reset Input
- Power Supply
- Ground
- Input
- Non-maskable Interrupt
- No Connect
- Active Low Reset Output
- Active High Reset Output
DESCRIPTION
The DS1707/DS1708 3.3- or 5.0-Volt MicroMonitor monitors three vital conditions for a microprocessor:
power supply, voltage sense, and external override. A precision temperature-compensated reference and
comparator circuit monitor the status of V
CC
at the device and at an upstream point for maximum
protection. When the sense input detects an out-of-tolerance condition a non-maskable interrupt is
generated. As the voltage at the device degrades an internal power fail signal is generated which forces
the reset 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 130 ms to allow the power supply and processor to stabilize.
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041305
DS1707/DS1708
The third function the DS1707/DS1708 performs is pushbutton reset control. The DS1707/DS1708
debounces the pushbutton input and guarantees an active reset pulse width of 130 ms minimum.
OPERATION
Power Monitor
The DS1707/DS1708 detects out-of-tolerance power supply conditions and warns a processor-based
system of impending power failure. When V
CC
falls below the minimum V
CC
tolerance, a comparator
outputs the RST and
RST
signals. RST and
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 and
RST
are kept
active for a minimum of 130 ms to allow the power supply and processor to stabilize.
Pushbutton Reset
The DS1707/DS1708 provides 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 RST and
RST
signals of at least 130 ms minimum will be generated. The 130 ms delay commences
as the pushbutton reset input is released from the low level. The pushbutton can be initiated by connecting
the
NMI
output to the
PBRST
input as shown in Figure 3.
Non-Maskable Interrupt
The DS1707/DS1708 generates a non-maskable interrupt (
NMI
) for early warning of a power failure. A
precision comparator monitors the voltage level at the IN pin relative to an on-chip reference generated
by an internal band gap. The IN pin is a high impedance input allowing for a user-defined sense point. An
external resistor voltage divider network (Figure 5) is used to interface with high voltage signals. This
sense point may be derived from a regulated supply or from a higher DC voltage level closer to the main
system power input. Since the IN trip point V
TP
is 1.25 volts, the proper values for R1 and R2 can be
determined by the equation as shown in Figure 5. Proper operation of the DS1707/DS1708 requires that
the voltage at the IN pin be limited to V
CC
. Therefore, the maximum allowable voltage at the supply being
monitored (V
MAX
) can also be derived as shown in Figure 5. A simple approach to solving the equation is
to select a value for R2 high enough to keep power consumption low, and solve for R1. The flexibility of
the IN input pin allows for detection of power loss at the earliest point in a power supply system,
maximizing the amount of time for system shut-down between
NMI
and RST/
RST
.
When the supply being monitored decays to the voltage sense point, the DS1707/DS1708 pulses the
NMI
output to the active state for a minimum 200
ms.
The
NMI
power-fail detection circuitry also has built-in
hysteresis of 100
mV.
The supply must be below the voltage sense point for approximately 5
ms
before a
low
NMI
will be generated. In this way, power supply noise is removed from the monitoring function,
preventing false interrupts. During a power-up, any detected IN pin levels below V
TP
by the comparator
are disabled from generating an interrupt until V
CC
rises to V
CCTP
. As a result, any potential
NMI
pulse
will not be initiated until V
CC
reaches V
CCTP
.
Connecting
NMI
to
PBRST
would allow the non-maskable interrupt to generate an automatic reset when
an out-of-tolerance condition occurred in a monitored supply. An example is shown in Figure 3.
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DS1707/DS1708
MICROMONITOR BLOCK DIAGRAM
Figure 1
40kW
PUSHBUTTON RESET
Figure 2
PUSHBUTTON RESET CONTROLLED BY
NMI
Figure 3
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DS1707/DS1708
TIMING DIAGRAM: PUSHBUTTON RESET
Figure 4
NON-MASKABLE INTERRUPT CIRCUIT EXAMPLE
Figure 5
V
SENSE
=
Example:
R1+ R2
X 1.25
R2
V
MAX
=
V
SENSE
X V
CC
V
TP
V
SENSE
= 4.50V at the trip point
V
CC
= 3.3V
10 kΩ = R2
4.50
X 3.3 = 11.88V maximum
1.25
Therefore:
4.50 =
R1
+
10k
X 1.25
10k
R1 = 26 kΩ
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微控制器 MCU
