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MAX6814XK-T
5-Pin Watchdog Timer Circuit
________________General DescriptionThe MAX6814 is a low-power watchdog circuit in a tiny 5-
pin SC70 package. This device improves system reliability
by monitoring the system for software code execution
errors. When the watchdog input detects a transitional
edge, the internal watchdog timer clears and restarts,
then begins counting again. If the watchdog timer
exceeds the watchdog timeout period (1.6s typ), the
active-low, push-pull watchdog output asserts for the
watchdog pulse period (140ms min) to alert the system of
the fault.
The MAX6814 consumes only 4µA of supply current and
is fully specified over the extended temperature range.
________________________ApplicationsComputers and Controllers
Embedded Controllers
Intelligent Instruments
Automotive Systems
Critical µP Monitoring
Features4µA Operating CurrentWatchdog Timer with 1.6s Timeout 140ms (min) WDOPulse Period Push-Pull Active-Low WDOFully Specified Over Extended Temperature
RangeNo External Components
MAX6814
5-Pin Watchdog Timer Circuit19-2804; Rev 1; 3/04
Ordering Information
Pin Configuration
Typical Operating Circuit
MAX6814
5-Pin Watchdog Timer Circuit
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= +2.25V to +5.5V, TA= TMINto TMAX, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VCC........................................................................-0.3V to +6.0V
All Other Pins..............................................-0.3V to (VCC+ 0.3V)
Input Current, WDI..............................................................20mA
Output Current, WDO..........................................................20mA
Continuous Power Dissipation (TA= +70°C)
5-Pin SC70 (derate 3.1mW/°C above +70°C)...............247mW
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Junction Temperature......................................................+150°C
Lead Temperature (soldering, 10s).................................+300°C
Note 1:Overtemperature limits are guaranteed by design, production testing performed at +25°C only.
Note 2:WDOis low when VCCfalls below the undervoltage threshold. When VCCrises above the undervoltage threshold, WDOgoes
high after the watchdog pulse period.
Note 3:Watchdog pulse period occurs when the watchdog times out or after VCCrises above the undervoltage threshold.
Note 4:The WDOshort-circuit current is the maximum pullup current when WDOis driven low.
Note 5:WDI is internally serviced within the watchdog period if WDI is left unconnected.
Note 6:The WDI input current is specified as the average input current when the WDI input is driven high or low. The WDI input is
designed to drive a three-stated output device with a 10µA maximum leakage current and a maximum capacitive load of
200pF. This output device must be able to source and sink at least 200µA when active.
MAX6814
5-Pin Watchdog Timer Circuit
__________________________________________Typical Operating Characteristics(VCC= +5V, TA= +25°C, unless otherwise noted.)
MAX6814
5-Pin Watchdog Timer CircuitFigure 1. Functional Diagram
_______________Detailed Description
Watchdog InputIn the MAX6814, the watchdog circuit monitors the µP’s
activity. If the µP does not toggle the watchdog input
(WDI) within tWD(1.6s), WDOasserts. The internal 1.6s
timer is cleared by either a WDOpulse or by toggling
WDI, which detects pulses as short as 50ns. While
WDOis asserted, the timer remains cleared and does
not count. As soon as WDOis released, the timer starts
counting (Figure 3).
Disable the watchdog function by leaving WDI uncon-
nected or by three-stating the driver connected to WDI.
The watchdog input is internally driven low during the
first 7/8 of the watchdog timeout period and high for the
last 1/8 of the watchdog timeout period. When WDI is left
unconnected, this internal driver clears the 1.6s timer
every 1.4s. When WDI is three-stated or unconnected,
the maximum allowable leakage current is 10µA and the
maximum allowable load capacitance is 200pF.
Applications Information
Watchdog Input CurrentThe MAX6814 WDI inputs are internally driven through
a buffer and series resistor from the watchdog counter
(Figure 1). When WDI is left unconnected, the watch-
dog timer is serviced within the watchdog timeout peri-
od by a low-high-low pulse from the counter chain. For
minimum watchdog input current (minimum overall
power consumption), leave WDI low for the majority of
the watchdog timeout period, pulsing it low-high-low
once within the first 7/8 of the watchdog timeout period
to clear the watchdog timer. If WDI is externally driven
high for the majority of the timeout period, up to 160µA
can flow into WDI.
MAX6814
5-Pin Watchdog Timer Circuit
____________________Chip InformationTRANSISTOR COUNT: 607
PROCESS: BiCMOS
Watchdog Software Considerations One way to help the watchdog timer monitor software
execution more closely is to set and clear the watchdog
input at different points in the program, rather than
pulsing the watchdog input high-low-high or low-high-
low. This technique avoids a stuck loop, in which the
watchdog timer would continue to be cleared inside the
loop, keeping the watchdog from timing out.
Figure 4 shows an example of a flow diagram where
the I/O driving the watchdog input is set high at the
beginning of the program, set low at the beginning of
every subroutine or loop, then set high again when the
program returns to the beginning. If the program
should hang in any subroutine, the problem would
quickly be corrected, since the I/O is continually set low
and the watchdog timer is allowed to time out, causing
an interrupt to be issued. This scheme results in higher
time average WDI input current than does leaving WDI
low for the majority of the timeout period and periodi-
cally pulsing it low-high-low (see the Watchdog Input
Currentsection).
Figure 4. Watchdog Flow Diagram
Figure 3. Watchdog Timing Relationship
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.
MAX6814
5-Pin Watchdog Timer Circuit
Package Information(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to /packages.)
