MAX6814XK

19-2804; Rev 2; 12/05 5-Pin Watchdog Timer Circuit 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 4µA of supply current and is fully specified over the extended temperature range. 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 Ordering Information ________________________Applications Computers and Controllers PART Embedded Controllers Intelligent Instruments TEMP RANGE PINPACKAGE TOP MARK MAX6814XK-T 5 SC70-5 AEK -40°C to +85°C Devices are available in both leaded and lead-free packaging. Specify lead-free by replacing “-T” with “+T” when ordering. Automotive Systems Critical µP Monitoring Pin Configuration Typical Operating Circuit TOP VIEW VCC VCC VCC WDO WDO 1 NMI GND 2 MAX6814 VCC 4 WDI MAX6814 μP WDI GND 5 N.C. 3 I/O GND SC70-5 ________________________________________________________________ Maxim Integrated Products 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. 1 MAX6814 ________________General Description MAX6814 5-Pin Watchdog Timer Circuit 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 = +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 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 = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1) PARAMETER Operating Voltage Range Supply Current Undervoltage Lockout Threshold Watchdog Pulse Period WDO Output Voltage WDO Output Short-Circuit Current SYMBOL VCC ISUPPLY WDI unconnected (Note 2) tPP (Note 3) UNITS 5.5 V 24 VCC = 2.5V 4 12 ISINK = 1.2mA, VCC = 2.1V tWDI MAX 10 ISOURCE = 30µA, VCC = 2.3V WDI Pulse Width TYP 2.25 VOL ISOURCE MIN VCC = 5.5V VOH tWD WDI Input Current (Note 6) TA = 0°C to +70°C UVLO Watchdog Timeout Period WDI Input Voltage (Note 5) CONDITIONS 2.19 140 200 0.8 ✕ VCC 0.3 1.12 VIL = 0.4V, VIH = 0.8 ✕ VCC V 280 VCC = 3.6V (Note 4) 1.60 µA s ns 0.7 ✕ VCC WDI = VCC, time average 120 -20 V 400 0.3 ✕ VCC WDI = 0, time average ms 2.40 50 VIL VIH µA -15 160 V 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. 2 _______________________________________________________________________________________ 5-Pin Watchdog Timer Circuit WDO PULSE PERIOD vs. TEMPERATURE 7 6 5 4 3 230 220 210 200 190 180 170 2 160 1 150 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (°C) 2.0 MAX6814 toc03 MAX6814 toc02 240 WDO PULSE PERIOD (ms) 8 SUPPLY CURRENT (mA) 250 MAX6814 toc01 9 WATCHDOG TIMEOUT PERIOD vs. TEMPERATURE 1.9 WATCHDOG TIMEOUT PERIOD (s) VCC SUPPLY CURRENT vs. TEMPERATURE 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) Pin Description PIN NAME FUNCTION 1 WDO 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 _______________________________________________________________________________________ 3 MAX6814 __________________________________________Typical Operating Characteristics (VCC = +5V, TA = +25°C, unless otherwise noted.) MAX6814 5-Pin Watchdog Timer Circuit MAX6814 VCC WDO PULSE GENERATOR UVLO WDI WATCHDOG TRANSITION DETECTOR WDO WATCHDOG TIMER GND Figure 1. Functional Diagram _______________Detailed Description Applications Information Watchdog Input Watchdog Input Current 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 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 10µA and the maximum allowable load capacitance is 200pF. 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. 4 _______________________________________________________________________________________ 5-Pin Watchdog Timer Circuit 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). VCC START VUVLO VUVLO SET WDI HIGH tPP WDO PROGRAM CODE Figure 2. Power-Up Timing Diagram SUBROUTINE OR PROGRAM LOOP SET WDI LOW VCC VUVLO WDO tPP tWD RETURN tPP Figure 4. Watchdog Flow Diagram WDI Figure 3. Watchdog Timing Relationship ____________________Chip Information TRANSISTOR COUNT: 607 PROCESS: BiCMOS _______________________________________________________________________________________ 5 MAX6814 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. 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 MAX6814 5-Pin Watchdog Timer Circuit PACKAGE OUTLINE, 5L SC70 21-0076 C 1 1 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.