________________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
Automotive Systems
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
MAX6814
5-Pin Watchdog Timer Circuit
________________________________________________________________ Maxim Integrated Products 1
19-2804; Rev 2; 12/05
Ordering Information
PART TEMP
RANGE
PIN-
PACKAGE
TOP
MARK
MAX6814XK-T
- 40°C to + 85°C
5 SC70-5 AE K
Pin Configuration
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Typical Operating Circuit
MAX6814 μP
VCC VCC
VCC
GND GND
NMI
I/O
WDO
WDI
15
2
34
GND
WDI
VCC
MAX6814
SC70-5
TOP VIEW
N.C.
WDO
Devices are available in both leaded and lead-free packaging.
Specify lead-free by replacing “-T” with “+T” when ordering.
MAX6814
5-Pin Watchdog Timer Circuit
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(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)
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
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
Operating Voltage Range VCC TA = 0°C to +70°C
2.25
5.5 V
VCC = 5.5V 10 24
Supply Current
ISUPPLY WDI unconnected
VCC = 2.5V 4 12 µA
Undervoltage Lockout Threshold UVLO (Note 2)
2.19
V
Watchdog Pulse Period tPP (Note 3)
140 200 280
ms
VOH ISOURCE = 30µA, VCC = 2.3V 0.8 ✕ VCC
WDO Output Voltage VOL ISINK = 1.2mA, VCC = 2.1V 0.3 V
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
VIL
0.3 ✕ VCC
WDI Input Voltage (Note 5) VIH 0.7 ✕ VCC
V
WDI = VCC, time average
120 160
WDI Input Current (Note 6) WDI = 0, time average -20
-15
µA
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.
MAX6814
5-Pin Watchdog Timer Circuit
_______________________________________________________________________________________ 3
Pin Description
__________________________________________Typical Operating Characteristics
(VCC = +5V, TA= +25°C, unless otherwise noted.)
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
250
150
-40 -20 40 100
WDO PULSE PERIOD
vs. TEMPERATURE
170
160
180
230
240
MAX6814 toc02
TEMPERATURE (°C)
WDO PULSE PERIOD (ms)
020 8060
210
220
190
200
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.
2GND Ground
3N.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.
5VCC Supply Voltage
MAX6814
5-Pin Watchdog Timer Circuit
4 _______________________________________________________________________________________
MAX6814
VCC
WDI
GND
WDO
WDO PULSE
GENERATOR
WATCHDOG
TIMER
WATCHDOG
TRANSITION
DETECTOR
UVLO
Figure 1. Functional Diagram
_______________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 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
_______________________________________________________________________________________ 5
____________________Chip Information
TRANSISTOR 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
Current section).
START
SET WDI
HIGH
PROGRAM
CODE
SUBROUTINE OR
PROGRAM LOOP
SET WDI LOW
RETURN
Figure 4. Watchdog Flow Diagram
VCC
VUVLO VUVLO
WDO tPP
Figure 2. Power-Up Timing Diagram
VCC
tWD tPP
tPP
VUVLO
WDI
WDO
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.
6___________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
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 www.maxim-ic.com/packages.)
SC70, 5L.EPS
PACKAGE OUTLINE, 5L SC70
21-0076 1
1
C