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MAX708 Schematic ( PDF Datasheet ) - Maxim Integrated

Teilenummer MAX708
Beschreibung Low-Cost / uP Supervisory Circuits
Hersteller Maxim Integrated
Logo Maxim Integrated Logo 




Gesamt 12 Seiten
MAX708 Datasheet, Funktion
19-4334; Rev. 5; 9/95
Low-Cost, µP Supervisory Circuits
_______________General Description
The MAX705-MAX708/MAX813L microprocessor (µP)
supervisory circuits reduce the complexity and number
of components required to monitor power-supply and
battery functions in µP systems. These devices signifi-
cantly improve system reliability and accuracy com-
pared to separate ICs or discrete components.
The MAX705/MAX706/MAX813L provide four functions:
1) A reset output during power-up, power-down, and
brownout conditions.
2) An independent watchdog output that goes low if
the watchdog input has not been toggled within 1.6
seconds.
3) A 1.25V threshold detector for power-fail warning,
low-battery detection, or for monitoring a power sup-
ply other than +5V.
4) An active-low manual-reset input.
The MAX707/MAX708 are the same as the MAX705/
MAX706, except an active-high reset is substituted for
the watchdog timer. The MAX813L is the same as the
MAX705, except RESET is provided instead of RESET.
Two supply-voltage monitor levels are available: The
MAX705/MAX707/MAX813L generate a reset pulse when
the supply voltage drops below 4.65V, while the
MAX706/MAX708 generate a reset pulse below 4.40V.
All four parts are available in 8-pin DIP, SO and µMAX
packages.
_______________________Applications
Computers
Controllers
Intelligent Instruments
Automotive Systems
Critical µP Power Monitoring
__________Typical Operating Circuit
UNREGULATED DC
MAX667
+5V DC LINEAR
REGULATOR
PUSHBUTTON
SWITCH
VCC RESET
PFI PFI
MAX705 WDO
MR
MAX706
MAX813L
PFO
µP
VCC
RESET
I/O LINE
NMI
INTERRUPT
___________________________Features
o µMAX Package: Smallest 8-Pin SO
o Guaranteed RESET Valid at VCC = 1V
o Precision Supply-Voltage Monitor
4.65V in MAX705/MAX707/MAX813L
4.40V in MAX706/MAX708
o 200ms Reset Pulse Width
o Debounced TTL/CMOS-Compatible
Manual-Reset Input
o Independent Watchdog Timer—1.6sec Timeout
(MAX705/MAX706)
o Active-High Reset Output
(MAX707/MAX708/MAX813L)
o Voltage Monitor for Power-Fail or Low-Battery
Warning
______________Ordering Information
PART
MAX705CPA
MAX705CSA
MAX705CUA
MAX705C/D
TEMP. RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
8 SO
8 µMAX
Dice*
Ordering Information continued at end of data sheet.
* Dice are specified at TA = +25°C.
**Contact factory for availability and processing to MIL-STD-883.
_________________Pin Configurations
TOP VIEW
MR 1
VCC 2
GND 3
PFI 4
MAX705
MAX706
MAX813L
8 WDO
7 RESET (RESET)
6 WDI
5 PFO
DIP/SO
MR 1
VCC 2
GND 3
PFI 4
MAX707
MAX708
8 RESET
7 RESET
6 N.C.
5 PFO
DIP/SO
( ) ARE FOR MAX813L ONLY.
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.






MAX708 Datasheet, Funktion
Low-Cost, µP Supervisory Circuits
6
WDI
WATCHDOG
TRANSITION
DETECTOR
VCC
MR 1
250µA
2
VCC
WATCHDOG
TIMER
TIMEBASE FOR
RESET AND
WATCHDOG
RESET
GENERATOR
8
WDO
7
RESET
(RESET)
4
PFI
4.65V*
MAX705
MAX706
MAX813L
5
PFO
1.25V
* 4.40V FOR MAX7O6.
( ) ARE FOR MAX813L ONLY.
3 GND
Figure 1. MAX705/MAX706/MAX813L Block Diagram
VCC
MR 1
250µA
2
VCC
4
PFI
4.65V*
RESET
GENERATOR
MAX707
MAX708
1.25V
* 4.40V FOR MAX7O6.
3 GND
Figure 2. MAX707/MAX708 Block Diagram
8
RESET
7
RESET
5
PFO
_______________Detailed Description
Reset Output
A microprocessor’s (µP’s) reset input starts the µP in a
known state. Whenever the µP is in an unknown state, it
should be held in reset. The MAX705-MAX708/MAX813L
assert reset during power-up and prevent code execu-
tion errors during power-down or brownout conditions.
On power-up, once VCC reaches 1V, RESET is a guaran-
teed logic low of 0.4V or less. As VCC rises, RESET stays
low. When VCC rises above the reset threshold, an inter-
nal timer releases RESET after about 200ms. RESET puls-
es low whenever VCC dips below the reset threshold, i.e.
brownout condition. If brownout occurs in the middle of
a previously initiated reset pulse, the pulse continues for
at least another 140ms. On power-down, once VCC falls
below the reset threshold, RESET stays low and is guar-
anteed to be 0.4V or less until VCC drops below 1V.
The MAX707/MAX708/MAX813L active-high RESET output
is simply the complement of the RESET output, and is
guaranteed to be valid with VCC down to 1.1V. Some µPs,
such as Intel’s 80C51, require an active-high reset pulse.
Watchdog Timer
The MAX705/MAX706/MAX813L watchdog circuit moni-
tors the µP’s activity. If the µP does not toggle the watch-
dog input (WDI) within 1.6sec and WDI is not three-stat-
ed, WDO goes low. As long as RESET is asserted or the
WDI input is three-stated, the watchdog timer will stay
cleared and will not count. As soon as reset is released
and WDI is driven high or low, the timer will start counting.
Pulses as short as 50ns can be detected.
Typically, WDO will be connected to the non-maskable
interrupt input (NMI) of a µP. When VCC drops below
the reset threshold, WDO will go low whether or not the
watchdog timer has timed out yet. Normally this would
trigger an NMI interrupt, but RESET goes low simultane-
ously, and thus overrides the NMI interrupt.
If WDI is left unconnected, WDO can be used as a low-
line output. Since floating WDI disables the internal
timer, WDO goes low only when VCC falls below the
reset threshold, thus functioning as a low-line output.
The MAX705/MAX706 have a watchdog timer and a
RESET output. The MAX707/MAX708 have both active-
high and active-low reset outputs. The MAX813L has
both an active-high reset output and a watchdog timer.
Manual Reset
The manual-reset input (MR) allows reset to be trig-
gered by a pushbutton switch. The switch is effectively
debounced by the 140ms minimum reset pulse width.
MR is TTL/CMOS logic compatible, so it can be driven
by an external logic line. MR can be used to force a
watchdog timeout to generate a reset pulse in the
MAX705/MAX706/MAX813L. Simply connect WDO to
MR.
6 _______________________________________________________________________________________

6 Page









MAX708 pdf, datenblatt
Low-Cost, µP Supervisory Circuits
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1995 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.

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