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19-2804; Rev 1; 3/04
5-Pin Watchdog Timer Circuit
________________General Description
The MAX6814 is a low-power watchdog circuit in a tiny 5pin 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 4A of supply current and is fully specified over the extended temperature range. 4A 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
Features
MAX6814
________________________Applications
Computers and Controllers Embedded Controllers Intelligent Instruments Automotive Systems Critical P Monitoring
MAX6814XK-T PART
Ordering Information
TEMP RANGE -40C to +85C PINPACKAGE 5 SC70-5 TOP MARK AEK
Typical Operating Circuit
VCC
Pin Configuration
TOP VIEW
VCC WDO NMI
VCC
WDO 1
5
VCC
MAX6814
WDI GND I/O
GND 2
MAX6814
P
N.C. 3 GND 4 WDI
SC70-5
________________________________________________________________ Maxim Integrated Products
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5-Pin Watchdog Timer Circuit MAX6814
ABSOLUTE MAXIMUM RATINGS
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 = +70C) 5-Pin SC70 (derate 3.1mW/C above +70C) ...............247mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C Junction Temperature ......................................................+150C Lead Temperature (soldering, 10s) .................................+300C
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.
ELECTRICAL CHARACTERISTICS
(VCC = +2.25V to +5.5V, TA = TMIN to TMAX, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER Operating Voltage Range Supply Current Undervoltage Lockout Threshold Watchdog Pulse Period WDO Output Voltage WDO Output Short-Circuit Current Watchdog Timeout Period WDI Pulse Width WDI Input Voltage (Note 5) WDI Input Current (Note 6) SYMBOL VCC ISUPPLY UVLO tPP VOH VOL ISOURCE tWD tWDI VIL VIH WDI = VCC, time average WDI = 0, time average -20 0.7 VCC 120 -15 160 VIL = 0.4V, VIH = 0.8 VCC CONDITIONS TA = 0C to +70C WDI unconnected (Note 2) (Note 3) ISOURCE = 30A, VCC = 2.3V ISINK = 1.2mA, VCC = 2.1V VCC = 3.6V (Note 4) 1.12 50 0.3 VCC 1.60 140 0.8 VCC 0.3 400 2.40 VCC = 5.5V VCC = 2.5V MIN 2.25 10 4 2.19 200 280 TYP MAX 5.5 24 12 UNITS V A V ms V A s ns V A
Note 1: Overtemperature limits are guaranteed by design, production testing performed at +25C 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 10A maximum leakage current and a maximum capacitive load of 200pF. This output device must be able to source and sink at least 200A when active.
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5-Pin Watchdog Timer Circuit MAX6814
__________________________________________Typical Operating Characteristics
(VCC = +5V, TA = +25C, unless otherwise noted.)
VCC SUPPLY CURRENT vs. TEMPERATURE
MAX6814 toc01
WDO PULSE PERIOD vs. TEMPERATURE
MAX6814 toc02
WATCHDOG TIMEOUT PERIOD vs. TEMPERATURE
1.9 WATCHDOG TIMEOUT PERIOD (s) 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0
MAX6814 toc03
9 8 SUPPLY CURRENT (mA) 7 6 5 4 3 2 1 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
250 240 WDO PULSE PERIOD (ms) 230 220 210 200 190 180 170 160 150 -40 -20 0 20 40 60 80
2.0
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
TEMPERATURE (C)
Pin Description
PIN 1 2 3 NAME WDO GND N.C. FUNCTION 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. Ground No Connection. Leave unconnected or connect to VCC. 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. Supply Voltage
4
WDI
5
VCC
_______________________________________________________________________________________
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5-Pin Watchdog Timer Circuit MAX6814
VCC
MAX6814
UVLO
WDO PULSE GENERATOR
WDO
WDI
WATCHDOG TRANSITION DETECTOR
WATCHDOG TIMER
GND
Figure 1. Functional Diagram
_______________Detailed Description
Watchdog Input
In the MAX6814, the watchdog circuit monitors the P's activity. If the P 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 unconnected 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 10A 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 160A can flow into WDI.
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_______________________________________________________________________________________
5-Pin Watchdog Timer Circuit
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-highlow. 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).
MAX6814
VCC VUVLO VUVLO
START
WDO
tPP
SET WDI HIGH PROGRAM CODE
Figure 2. Power-Up Timing Diagram
SUBROUTINE OR PROGRAM LOOP SET WDI LOW
VCC VUVLO WDO tPP tWD tPP RETURN
WDI
Figure 4. Watchdog Flow Diagram
Figure 3. Watchdog Timing Relationship
____________________Chip Information
TRANSISTOR COUNT: 607 PROCESS: BiCMOS
_______________________________________________________________________________________
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5-Pin Watchdog Timer Circuit MAX6814
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
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 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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