General Description
The 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.
Applications
●Computers and Controllers
●Embedded Controllers
●Intelligent Instruments
●Critical µP Monitoring
Features
●4µA Operating Current
●Watchdog Timer with 1.6s Timeout
●140ms (min) WDO Pulse Period
●Push-Pull Active-Low WDO
●Fully Specified Over Extended Temperature Range
●No External Components
Devices are available in both leaded and lead(Pb)-free/RoHS-
compliant packaging. Specify lead-free by replacing “-T” with
“+T” when ordering.
PART TEMP
RANGE
PIN-
PACKAGE
TOP
MARK
MAX6814XK-T -40°C to +85°C 5 SC70 AEK
15
2
34
GND
WDI
VCC
MAX6814
SC70-5
TOP VIEW
N.C.
WDO
MAX6814
µP
VCC VCC
VCC
GND GND
NMI
I/O
WDO
WDI
MAX6814 5-Pin Watchdog Timer Circuit
19-2804; Rev 3; 7/14
Pin CongurationTypical Operating Circuit
Ordering Information
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
(VCC = +2.25V to +5.5V, TA = TMIN to TMAX, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
(Note 1)
Note 1: Overtemperature limits are guaranteed by design, production testing performed at +25°C only.
Note 2: WDO is low when VCC falls below the undervoltage threshold. When VCC rises above the undervoltage threshold, WDO goes
high after the watchdog pulse period.
Note 3: Watchdog pulse period occurs when the watchdog times out or after VCC rises above the undervoltage threshold.
Note 4: The WDO short-circuit current is the maximum pullup current when WDO is 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.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Operating Voltage Range VCC TA = 0°C to +70°C 2.25 5.5 V
Supply Current ISUPPLY WDI unconnected VCC = 5.5V 10 24 µA
VCC = 2.5V 4 12
Undervoltage Lockout ThresholdUVLO (Note 2) 2.19 V
Watchdog Pulse Period tPP (Note 3) 140 200 280 ms
WDO Output Voltage VOH ISOURCE = 30µA, VCC = 2.3V 0.8 × VCC V
VOL ISINK = 1.2mA, VCC = 2.1V 0.3
WDO Output Short-Circuit Current ISOURCE VCC = 3.6V (Note 4) 400 µA
Watchdog Timeout Period tWD 1.12 1.60 2.40 s
WDI Pulse Width tWDI VIL = 0.4V, VIH = 0.8 × VCC 50 ns
WDI Input Voltage (Note 5) VIL 0.3 × VCC V
VIH 0.7 × VCC
WDI Input Current (Note 6) WDI = VCC, time average 120 160 µA
WDI = 0, time average -20 -15
MAX6814 5-Pin Watchdog Timer Circuit
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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.
Absolute Maximum Ratings
Electrical Characteristics
(VCC = +5V, TA = +25°C, unless otherwise noted.)
PIN NAME FUNCTION
1WDO Active-Low Watchdog Output. Pulses low for 140ms (min) when the watchdog timer exceeds the watchdog
timeout period. WDO is low when VCC is below the UVLO threshold and remains low for 140ms (min) after
VCC exceeds the UVLO threshold.
2 GND Ground
3 N.C. No Connection. Leave unconnected or connect to VCC.
4 WDI
Watchdog Input. If WDI remains either high or low for longer than the watchdog timeout period, the internal
watchdog timer runs out and a watchdog pulse period is triggered. The internal watchdog timer clears
whenever a watchdog pulse period is asserted, or whenever WDI sees a rising or falling edge. If WDI is left
unconnected or is connected to a three-stated buffer output, the watchdog is disabled.
5 VCC Supply Voltage
2.0
1.0
-40 -20 40 100
WATCHDOG TIMEOUT PERIOD
vs. TEMPERATURE
1.2
1.1
1.3
1.8
1.9
MAX6814 toc03
TEMPERATURE (°C)
WATCHDOG TIMEOUT PERIOD (s)
0 20 8060
1.6
1.7
1.4
1.5
250
150
-40 -20 40 100
WDO PULSE PERIOD
vs. TEMPERATURE
170
160
180
230
240
MAX6814 toc02
TEMPERATURE (°C)
WDO PULSE PERIOD (ms)
0 20 8060
210
220
190
200
1
3
2
6
5
4
8
7
9
-40 20 40-20 0 60 80 100 120
MAX6814 toc01
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
VCC SUPPLY CURRENT
vs. TEMPERATURE
MAX6814 5-Pin Watchdog Timer Circuit
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Pin Description
Typical Operating Characteristics
Detailed Description
Watchdog Input
In the MAX6814, the watchdog circuit monitors the µP’s
activity. If the µP does not toggle the watchdog input
(WDI) within tWD (1.6s), WDO asserts. The internal 1.6s
timer is cleared by either a WDO pulse or by toggling
WDI, which detects pulses as short as 50ns. While
WDO is asserted, the timer remains cleared and does
not count. As soon as WDO is 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 Current
The MAX6814 WDI inputs are internally driven through
a buffer and series resistor from the watchdog counter
(Figure 1). When WDI is left unconnected, the watchdog
timer is serviced within the watchdog timeout period
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.
Figure 1. Functional Diagram
MAX6814
VCC
WDI
GND
WDO
WDO PULSE
GENERATOR
WATCHDOG
TIMER
WATCHDOG
TRANSITION
DETECTOR
UVLO
MAX6814 5-Pin Watchdog Timer Circuit
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Watchdog Software Considerations
One way to help the watchdog timer monitor software
execution more closely is to set and clear the watch-
dog 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 periodically pulsing it
low-high-low (see the Watchdog Input Current section).
Figure 4. Watchdog Flow Diagram
Figure 3. Watchdog Timing Relationship
Figure 2. Power-Up Timing Diagram
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
5 SC70 X5+1 21-0076 90-0188
START
SET WDI
HIGH
PROGRAM
CODE
SUBROUTINE OR
PROGRAM LOOP
SET WDI LOW
RETURN
VCC
tWD tPP
tPP
VUVLO
WDI
WDO
VCC
VUVLO VUVLO
WDO tPP
MAX6814 5-Pin Watchdog Timer Circuit
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Chip Information
TRANSISTOR COUNT: 607
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
3 7/14 No /V OPNs; removed Automotive reference from Applications section 1
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
MAX6814 5-Pin Watchdog Timer Circuit
© 2014 Maxim Integrated Products, Inc.
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Revision History
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
