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_______________general description the max814/max815/max816 are high-accuracy microprocessor (p) supervisory circuits that provide power-on reset, watchdog, and power-fail functions. they eliminate manual trimming and improve reliability in critical applications needing high-accuracy reset thresholds. the reset output is guaranteed to be in the correct state for v cc down to 1v. the reset com- parator is designed to ignore fast transients on v cc . reset thresholds are available for operation with a vari-ety of 3v and 5v supply voltages. a 75? maximum supply current makes the max814/ max815/max816 ideal for use in portable equipment. all three devices are available in 8-pin dip and so pack- ages. see the selector table for a review of features. ________________________applications medical equipmentcontrollers intelligent instruments critical ? power monitoring portable/battery-powered equipment set-top boxes ____________________________features ? 1% worst-case reset threshold accuracy ? 4.8v, 4.7v, 4.55v, 3.03v, or adjustable resetthresholds ? 1% low-line threshold accuracy (max814) 60mv above reset threshold ? 200ms reset time delay ? active-low reset output active-high reset output (max814/max816) ? 75a max supply current ? guaranteed reset valid to v cc = 1v ? manual reset input ? 2% power-fail comparator ? independent watchdog with 1.56sec timeout(max815) ? power-supply glitch immunity ? 8-pin so and dip packages max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits ________________________________________________________________ maxim integrated products 1 low line gnd pfo pfi 12 8 7 resetreset v cc mr max814 dip/so top view 34 65 wdi gnd pfo pfi 12 8 7 wdoreset v cc mr max815 dip/so 34 65 reset in gnd pfo pfi 12 8 7 resetreset v cc mr max816 dip/so 34 65 __________________________________________________________pin configurations 19-0412; rev 1; 12/05 _____________________selector table reset trip thresholds table appears at end of data sheet. ordering information continued at end of data sheet. feature max814 max815 max816 reset output ? ? ? reset output ? ? manual reset ? ? ? v cc reset voltage k, l, n, t k, l, n, t adjustable power-fail monitor ? ? ? low-line detector ? watchdog circuit ? for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part* temp range pin-package max814 _cpa 0? to +70? 8 pdip ______________ordering information * the max814/max815 offer a choice of reset threshold voltages. from the reset trip threshold table, select the suffix corre- sponding to the desired threshold and insert it into the blank to complete the part number. devices are available in both leaded and lead-free packaging. specify lead free by adding the + symbol at the end of the part number when ordering. downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics, +5v parts (max814/max815k, l, n)(v cc = 4.85v to 5.5v for max814k/max815k, v cc = 4.75v to 5.5v for max814l/max815l, v cc = 4.60v to 5.5v for max814n/ max815n, t a = t min to t max , unless otherwise noted.) stresses beyond those listed under ?bsolute 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. terminal voltage (with respect gnd) v cc ....................................................................-0.3v to +6.0v all other pins (note 1)........................... -0.3v to (v cc + 0.3v) terminal current (pfi, reset in, mr ) ................................10ma terminal current (all other pins) .........................................20ma continuous power dissipation (t a = +25?) plastic dip (derate 9.09mw/? above +70?) ............727mw so (derate 5.88mw/? above +70?) .........................471mw operating temperature ranges commercial.........................................................0? to +70? extended ...........................................................-40? to +85c storage temperature range .............................-65? to +125? max814k, v cc rising max814_e, v cc falling max814_c, v cc falling max814_c, max815_c max815 (note 1) wdi = v cc or wdi = 0v max814_e, max815_e max815, v cc = 5.0v max814_c, max815_c max815 max815 max814 max814_e/max815_e, v cc = 1.2v, i sink = 100? max814_c/max815_c, v cc = 1.0v, i sink = 50? max814_e, max815_e max814k, max815k max814l, max815l i sink = 3.2ma max814n, max815n i source = 800? conditions 4.93 mv 48 73 ? v ll low line to reset differential threshold 50 70 ? 1 t wdo mr to wdo high delay ? -1.0 1.0 i wdi wdi input current v 2.4 v wdi wdi input threshold 0.8 ns 50 t wp wdi pulse width s 1.12 2.00 t wd watchdog timeout period v 0.3 reset , wdo , pfo , low line output voltage 0.3 0.4 v ol v cc -1.5 v oh 75 v 1.2 5.5 v cc 1.0 5.5 operating voltage range v 0.4 v ol reset output voltage v cc -1.5 v oh ms 140 200 250 t rs reset pulse width mv 0 reset threshold hysteresis ? 85 i supply supply current 4.75 4.85 4.65 4.75 v 4.50 4.60 v rt reset threshold units min typ max symbol parameter max814l, v cc rising 4.83 v llt max814n, v cc rising 4.68 v low line threshold i source = 800? i sink = 3.2ma low high downloaded from: http:///
i sink = 3.2ma max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits _______________________________________________________________________________________ 3 electrical characteristics, +5v parts (max814/max815k, l, n) (continued)(v cc = 4.85v to 5.5v for max814k/max815k, v cc = 4.75v to 5.5v for max814l/max815l, v cc = 4.60v to 5.5v for max814n/ max815n, t a = t min to t max , unless otherwise noted.) electrical characteristics, +3v parts (max814/max815t, max816)(v cc = 3.06v to 5.5v for max814t/max815t and max816, t a = t min to t max , unless otherwise noted.) 0? to +70? -40? to +85? 0? to +70? max815t t a = -40? to +85?, v cc = 1.2v, i sink = 100? t a = 0? to +70?, v cc = 1.0v, i sink = 50? max816 -40? to +85? 0? to +70? -40? to +85? conditions i source = 500? s 1.12 2.00 t wd watchdog timeout period 0.3 i sink = 1.2ma 0.3 0.3 v ol 0.8 x v cc v oh 75 v 1.2 5.5 v cc 1.0 5.5 operating voltage range 0.3 v ol 0.8 x v cc v oh ms 140 200 250 t rs reset pulse width na -15 +6 +35 i rt reset in input current ? 85 i supply supply current 3.00 3.06 3.00 3.08 v v rt reset threshold units min typ max symbol parameter max816c max816e 1.683 1.700 1.717 1.678 1.700 1.722 v v rit reset in threshold mv 0 reset threshold hysteresis i source = 800? v 0.4 v ol reset output voltage v cc -1.5 v oh i source = 500? i sink = 1.2ma v ol reset , wdo , pfo , low line output voltage v 0.4 v cc -1.5 v oh i source = 800? i sink = 3.2ma v ol conditions mr input threshold v mr hi high 0.7 x v cc v mr to reset out delay t md (note 3) 250 ns pfi input threshold v pfi v cc = 5.0v 2.45 2.50 2.55 v pfi input current i pfi units min typ max symbol parameter mr pull-up current i mr mr = 0v 70 240 ? mr pulse width t mr 150 ns v mr lo low 1.1 -15.00 +6.0 +35.00 na low line , pfo , wdo assertion delay (note 2) 200 ? v rt (max) < v cc <3.6 v; max814t, max816 4.5v < v cc < 5.5v; max814t, max816 v rt (max) < v cc < 3.6v 4.5v < v cc < 5.5v downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits 4 _______________________________________________________________________________________ electrical characteristics, +3v parts (max814/max815t, max816) (continued)(v cc = 3.06v to 5.5v for max814t/max815t and max816, t a = t min to t max , unless otherwise noted.) v cc rising v cc falling, max814te v cc falling, max814tc max815t (note 1) wdi = v cc or 0v, max815t v rt (max) < v cc < 3.6v; max815t max815t low conditions 3.163 mr input threshold mv 48 73 ? v ll low line to reset differential threshold 50 70 v mr hi high ? 1 t wdo mr to wdo high delay ? -1.0 +1.0 i wdi wdi input current v 0.7 x v cc 0.8 0.7 x v cc v 100 t wp wdi pulse width high mr to reset out delay (note 3) t md v rt (max) < v cc < 3.6v 750 pfi input threshold v pfi v cc = 3.3v, 5v 1.666 1.700 1.734 v pfi input current i pfi units min typ max symbol parameter v llt v mr pullup current i mr mr = 0v 70 240 ? mr pulse width t mr v rt (max) < v cc < 3.6v 500 ns v mr lo low 1.1 -15.00 +6.0 +35.00 na low line threshold 50 4.5v < v cc < 5.5v v rt (max) < v cc < 3.6v v cc = 5.0v; max815t low 2.4 v wdi wdi input threshold 0.8 high 110 370 4.5v < v cc < 5.5v 150 4.5v < v cc < 5.5v 250 ns v rt (max) < v cc < 3.6v 4.5v < v cc < 5.5v note 1: applies if wdo is externally connected to mr or if mr is externally driven. note 2: on power-up, delay from reset trip threshold crossing to valid outputs. note 3: applies to both reset and reset. ns downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits _______________________________________________________________________________________ 5 7030 -60 -40 -20 40 100 v cc supply current vs. temperature 3v parts (max814t/max815t, max816) 4540 35 65 max814-01 temperature ( c) v cc supply current ( a) 020 6080 6055 50 v cc = 5.0v v cc = 3.3v v cc = 2.5v 70 30 -60 -40 -20 40 100 v cc supply current vs. temperature 5v parts (max814/max815k, l, n) 4540 35 65 max814-02 temperature ( c) v cc supply current ( a) 020 6080 6055 50 v cc = 5.0v v cc = 3.3v 200175 -60 -40 -20 40 100 reset timeout period vs. temperature 185180 max814-03 temperature ( c) reset timeout period (ms) 020 6080 195190 v cc = 5.0v v cc = 3.3v 4015 -60 -40 -20 40 100 reset-comparator propagation delay vs. temperature 2520 max814-04 temperature ( c) propagation delay ( s) 020 6080 3530 100mv overdrive(v rt - v cc ) 1.7151.670 -60 -40 -20 40 100 pfi threshold vs. temperature 3v parts (max814t/max815t, max816) 1.6851.680 1.675 1.710 max814-05 temperature ( c) pfi threshold (v) 020 6080 1.7051.700 1.695 1.690 2.5502.450 -60 -40 -20 40 100 pfi threshold vs. temperature 5v parts (max814/max815k, l, n) 2.4902.470 2.4802.460 2.540 max814-06 temperature ( c) pfi threshold (v) 020 6080 2.5302.520 2.500 2.510 __________________________________________typical operating characteristics (t a = +25?, unless otherwise noted.) 60 0 -60 -40 -20 40 100 low-line comparator propagation delay vs. temperature 3020 10 max814-11 temperature ( c) propagation delay ( s) 020 6080 5040 100mv overdrive(v llt - v cc ) 7050 -60 -40 -20 40 100 low-line to reset threshold vs. temperature (v cc rising) 5854 5652 68 max814-10 temperature ( c) low-line to reset threshold (mv) 020 6080 6664 60 62 70 50 -60 -40 -20 40 100 low-line to reset threshold vs. temperature (v cc falling) 5854 5652 68 max814-09 temperature ( c) low-line to reset threshold (mv) 020 6080 6664 60 62 downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits 6 _______________________________________________________________________________________ 2 -3 -60 -40 -20 40 100 reset threshold deviation vs. temperature 3v parts (max814t/max815t, max816) -1-2 max814-07 temperature ( c) reset threshold deviation (mv) 020 6080 10 140 0 10 100 1000 maximum transient duration vs. reset comparator overdrive 5v parts (max814/max815k, l, n) 20 max814-13 reset comparator overdrive (mv), v rt - v cc maximum transient duration ( s) 8060 40 100 120 reset occurs above curve ____________________________typical operating characteristics (continued) (t a = +25?, unless otherwise noted.) 2 -3 -60 -40 -20 40 100 reset threshold deviation vs. temperature 3v parts (max814t/max815t, max816) -1-2 max814-07 temperature ( c) reset threshold deviation (mv) 020 6080 10 1 -2 -60 -40 -20 40 100 reset threshold deviation vs. temperature 5v parts (max814/max815k, l, n) -1 max814-08 temperature ( c) reset threshold deviation (mv) 020 6080 0 downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits _______________________________________________________________________________________ 7 _______________detailed description the max814/max815/max816 are high-accuracy, low- power microprocessor (p) supervisory circuits. they have ?-reset, watchdog-timer, and power-fail func- tions. typical applications illustrating their similarities and differences are shown in figures 1, 2, and 3. figures 4, 5, and 6 show the block diagrams of these parts. ______________________________________________________________pin description pin max814 max815 max816 name function 1 1 1 mr manual-reset input. triggers a reset when pulled below 1.10v. this active-low input has an internal 150? pullup current to v cc , and can be driven with cmos logic or shorted to gnd with a switch or transistor. 2 2 positive power-supply input. when v cc is below the reset threshold voltage*, reset is low, and remains low for a minimum of 140ms after it rises above the threshold. 2 v cc positive power-supply input. on the max816, reset is controlled by reset in, not v cc . 3 3 3 gnd ground 4 4 4 pfi power-fail input. the pfi threshold voltage is 1.70v on the max816 and onmax814/max815 parts with the t suffix. it is 2.50v on max814/max815 parts with k, l, and n suffixes. connect pfi to gnd or v cc when not used. 5 5 5 pfo power-fail output. when pfi is below its threshold, pfo is low; otherwise it is high. 6 wdi watchdog cmos input. if wdi remains high or low for more than 1.56s, thewatchdog timer times out, and wdo goes low. the timer is reset to zero on each wdi transition. 6 low line low-line output. normally high, low line goes low when v cc falls 60mv above the reset threshold. it returns high as soon as v cc rises above the low-line threshold. 6 reset in reset comparator input. reference is 1.70v. when reset in is below1.70v, reset is low, and remains low for a minimum of 140ms after it rises above the reference. 7 7 7 reset reset output. normally high, active low. controlled by mr and reset comparator. 8 8 reset reset output. active high. the inverse of reset . 8 wdo watchdog output. normally high, wdo goes low whenever the v cc reset threshold comparator input voltage is low or when the watchdog timertimes out. there is no appreciable delay going either direction when the v cc threshold comparator toggles. *reset threshold voltage is determined by part number suffix: k = 4.80v, l = 4.70v, n = 4.55v, t = 3.03v. downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits 8 _______________________________________________________________________________________ max814k/l/n v cc +5v +3.3v pfi mrpfo gnd reset reset max814 v cc v cc +12v supply gnd manual reset input pfimr reset p resetnmi interrupt low line pfo reset max816 v cc v cc v rt +12v supply gnd manual reset input pfi on power-up, reset is low until v cc > 2.8v. r1r2 mr reset in p reset interrupt reset pfo reset max815 v cc v cc +12v supply gnd manual reset input pfimr reset p reseti/o line nmi interrupt wdi wdo pfo figure 1a. typical application for dual +3.3v and +5v systems figure 1b. max814 typical application figure 2. max815 typical application figure 3. max816 typical application downloaded from: http:///
reset output a ?? reset input starts the ? in a known state. whenever the ? is in an unknown state, it should be held in reset. the max814/max815/max816 assert reset during power-up, power-down, or brownout conditions. on power-up, once v cc reaches 1v, reset is a guar- anteed logic low of 0.4v or less. as v cc rises, reset stays low. as v cc rises above the reset threshold, an internal timer releases reset after 200ms. reset also pulses low whenever v cc dips below the reset thresh- old (i.e., brownout condition). if brownout occurs in themiddle of a previously initiated reset, the internal timer is reset and the output remains low for at least another 140ms after the brownout ends. on power-down, once v cc falls below the reset threshold, reset stays low and is guaranteed to be less than 0.3v until v cc drops below 1v.the max814 and max816 also offer active-high reset outputs. they are the inverse of the reset outputs. max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits _______________________________________________________________________________________ 9 max814 reset generator reset threshold power- fail threshold 60mv 1 150 a 87 6 5 mr resetreset low line pfo v cc pfi 24 gnd 3 max815 timebase reset threshold power-fail threshold 150 a 6 87 5 mr v cc wdoreset pfo wdi pfi 2 14 gnd 3 reset generator watchdog timer watchdog transition detector max816 reset generator 1.70v 1.70v 1 2 150 a 87 5 mr v cc resetreset pfo reset in pfi 64 gnd 3 figure 4. max814 block diagram figure 5. max815 block diagram figure 6. max816 block diagram downloaded from: http:///
max814/max815/max816 reset threshold the max814/max815 have fixed, factory-set resetthresholds, signified by the first suffix letter in the part number (see figure 7 for more information on resetranges). the max816 has an adjustable reset threshold. max814/max815 k-suffix parts have a minimum reset threshold set to 4.75v, worst case. they are intended for 5.0v systems with a ?% or better power-supply tol- erance design that must meet worst-case system para- meters over time, temperature, line, and load variations. typically, the reset threshold (v rt ) is greater than or equal to the minimum ic operating voltage (v icmin ). the ??series 1%-tolerance reset threshold allows alarger range of power-supply tolerance. system ics that have a tight operating supply range, like the386/486 ?s, need a reset initiated at a minimum threshold of 4.75v, worst case. l-suffix parts have a minimum reset threshold set to 4.65v, worst case. they are intended for 5.0v systems with a ?% power-supply tolerance. typically, the reset threshold is less than or equal to the minimum power- supply voltage, allowing system operation over the complete power-supply range. a reset is initiated at 4.75v maximum. the 1% ??version maximizes the system ic guard-band range. n-suffix parts have a minimum reset threshold set to 4.50v, worst case. they are intended for 5.0v systems with a ?0% ic system. typically, the reset threshold 1% accuracy, low-power, +3v and +5v p supervisory circuits 10 ______________________________________________________________________________________ 4.5 4.75 4.85 5.0 5.25 5.5v ic operating rangereset limits ? (4.8v ?%) power-supply range(5.05v ?%) v rt > v ic(min) designed to meet worst-case design parameters. 4.5 4.75 4.65 5.0 5.25 5.5v ic operating rangereset limits ? (4.7v ?%) power-supply range(5v ?%) v rt < v ps(min) designed to allow operationover the full power-supply range. 4.5 4.75 4.60 5.0 5.25 5.5v ic operating rangereset limits ? (4.55v ?%) power-supply range(5v ?%) v rt > v ic(min) designed to meet worst-case design parameters. 3.0 3.06 3.3 3.6v ic operating rangereset limits ? (3.03v ?%) power-supply range(3.33v ?.26%) v rt > v ic(min) designed to meet worst-case design parameters. figure 7a. k suffix design range figure 7b. l suffix design range figure 7c. n suffix design range figure 7d. t suffix design range downloaded from: http:///
(v rt ) is greater than or equal to the minimum ic operat- ing voltages (v icmin ). the 1% ??series allows the use of a 5v ?% power supply, and guarantees systemoperation over worst-case conditions, maximizing the power-supply guard-band range. t-suffix parts have a minimum reset threshold set to 3.00v, worst case. they are intended for 3.3v systems (3.33v ?.26v) with a 7.8% or better power-supply tol- erance. typically, the reset threshold (v rt ) is greater than or equal to the minimum ic operating voltages(v icmin ). the max816 has an adjustable reset threshold, set withan external resistive divider (figure 3). the voltage on the reset in pin is monitored, not the voltage on v cc . the reset in threshold is 1.700v, and has very highimpedance and 35na maximum leakage. calculate the trip point, v rt , as follows: where v rt = the desired reset threshold, v rit is the reset in threshold (1.700v), r1 is the resistor con- nected between v rt and reset in, and r2 is the resistor connected between reset in and gnd.resistors r1 and r2 can have very high values. the usual procedure is to set r2 to some conveniently high value (100k ? , for example) and calculate r1 based on the desired reset threshold, using the following formula:the max816 can achieve ?.2% accuracy with 0.1% resistors. watchdog timer (max815) the watchdog circuit monitors the ?? activity. if the ? does not toggle the watchdog input (wdi) within the watchdog timeout period (t wp ), wdo goes low (figure 8). wdo also goes low during reset conditions. whenever v cc is below the reset threshold, wdo stays low; however, unlike reset , wdo does not have a mini- mum pulse width. as soon as v cc rises above the reset threshold, wdo goes high with no delay (figure 9). typically, wdo is connected to the non-maskable inter- rupt (nmi) of a ?. when v cc drops below the reset threshold, wdo goes low whether or not the watchdog timer has timed out (figure 9). this would normally trig- ger an nmi interrupt, but reset goes low simultane- ously and thus overrides the nmi interrupt.connecting wdo to mr enables the watchdog timeout to generate a reset in the max815. early power-fail warning critical systems often require early warning to indicatewhen power is failing. this warning provides time for the ? to store vital data and take care of any additional ?ousekeeping?before the power supply gets too far out of tolerance for the ? to operate reliably. power-fail comparator the power-fail comparator is intended as an undervolt-age detector to signal a failing power supply. however, the comparator does not need to be dedicated to this function, because it is completely separate from the rest of the circuitry. to build an early-warning circuit for power failure, connect the pfi pin to a voltage divider (see figures 1, 2, and 3). choose the voltage divider ratio, so the voltage at pfi falls below v pfi just before the monitored voltage drops out. use pfo to interrupt the ?, so it can prepare for an orderly power-down.the power-fail input (pfi) is compared to an internal reference. if the voltage on pfi is less than the power- fail reference, pfo sinks at least 1.2ma to gnd; other- wise it sources at least 300? from v cc . the reference is 2.50v in the max814/max815 with k, l, n suffixes, or1.70v with the t suffix. it is also 1.70v in the max816. low line output (max814) the low-line detector is a separate comparator thatmonitors v cc with a typical threshold voltage of 60mv above the normal reset threshold, with 2mv of hystere-sis (figure 9). if v cc rises faster than 10?/v, insert a 100pf capacitor from low line to gnd to ensure proper start-up. for normal operation (v cc above the reset threshold), low line is pulled to v cc . use low line to provide an nmi to the ? when power begins to fall. in most battery-operated portable systems, reserveenergy in the battery provides ample time to complete the shutdown routine once the low-line warning is encountered, and before reset asserts. if the system must also contend with a more rapid v cc fall time such as when the main battery is disconnected or ahigh-side switch is opened during operation?se capacitance on the v cc line to provide time to execute the shutdown routine. first, calculate the worst-casetime required for the system to perform its shutdown routine. then use the worst-case shutdown time (t shdn ), worst-case load current (i load ), and minimum low-line to reset threshold (v lr ) to calculate the amount of capacitance required to allow the shutdown routineto complete before reset is asserted. c it v hold load shdn lr = rr v v rt rit 12 1 = () [] / v vrr r rt rit = + () 12 2 max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits ______________________________________________________________________________________ 11 downloaded from: http:///
max814/max815/max816 where c hold is the capacitance (in farads), i load is the current being drained from the capacitor (inamperes), and v lr is the low-line to reset threshold dif- ference (in volts). manual reset many ?-based products require manual-reset capabil-ity, allowing the operator, a test technician, or external logic circuitry to initiate a reset. a logic low on mr asserts reset. reset remains asserted while mr is low, and for t rs (200ms) after mr returns high. this input has an internal pullup resistor, so it can be left open ifnot used. mr can be driven with ttl/cmos-logic lev- els or with open-drain/collector outputs.connect a normally open momentary switch from mr to gnd to create a manual-reset function; externaldebounce circuitry is not required. the watchdog circuit can be used to force a reset in the max815 by connecting wdo to mr . if mr is driven from long cables, or the device is used in a noisy envi-ronment, connect a 0.1? capacitor to ground to pro- vide additional noise immunity. __________applications information low-voltage operation the low line , pfo , and wdo outputs will be locked to logic low when the power supply drops below the lock- out threshold (typically 1v below the reset threshold). ensuring a valid reset output down to v cc = 0v when v cc falls below 1v, the reset output no longer sinks current, but becomes an open circuit. high- impedance cmos-logic inputs can drift to undeter- mined voltages if left undriven. if a pulldown resistor is added to the reset pin as shown in figure 10, any stray charge or leakage currents will be drained to ground, holding reset low. resistor value r1 is not critical. it should be about 100k ? ?arge enough not to load reset , and small enough to pull reset to ground. 1% accuracy, low-power, +3v and +5v p supervisory circuits 12 ______________________________________________________________________________________ figure 9. timing diagram ? v ll v cc 00 0 v cc v cc v cc v llt 60mv v rt v rt t rs t mr t md t rs low line(max814) wdo (max815) mr reset figure 8. max815 watchdog timing v cc 0v v cc 0v v cc 0v v cc 0v mr wdi wdo resetreset t wd t wp t wd t wd t wdo downloaded from: http:///
monitoring voltages other than v cc monitor voltages other than the v cc by connecting a voltage divider to pfi and adjusting the ratio appropri-ately. if required, add hysteresis by connecting a resis- tor (with a value approximately 10-times the sum of the two resistors in the potential divider network) between pfi and pfo . a capacitor between pfi and gnd will reduce the power-fail circuit? sensitivity to high-fre-quency noise on the line being monitored. reset can be asserted on other voltages in addition to the +5vv cc line. connect pfo to mr to initiate a reset when pfi drops below 2.50v (k, l, n suffix) or 1.70v (t suffixor max816). figure 11 shows the max814k/l/n/ max815k/l/n configured to assert reset when the +5v supply falls below the reset threshold, or when the+12v supply falls below approximately 11v. monitoring a negative voltage the power-fail comparator can also monitor a negativesupply rail (figure 12). when the negative rail is good (a negative voltage of large magnitude), pfo is low. when the negative rail is degraded (a negative voltage of lesser magnitude), pfo is high. by adding the resis- tors and transistor as shown, a high pfo triggers reset. as long as pfo remains high, the max814/max815/ max816 will keep reset asserted (reset = low, reset = high). note that this circuit? accuracy depends on thepfi threshold tolerance, the v cc line, and the resistor. watchdog software considerations a way to help the watchdog timer keep closer tabs onsoftware execution involves setting and resetting 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 where the watchdog timer continues to be reset within the loop, keeping the watchdog from timing out. figure 13 shows an example flow diagram where the i/o driving the watchdog input is set low at the beginning of the program, set high at the beginning of every subroutine, then set low at the end of every subroutine. if the pro- gram should hang in any subroutine, the i/o is continu- ally set high and the watchdog timer is allowed to time out, causing a reset to be issued. max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits ______________________________________________________________________________________ 13 max814max815 max816 reset r1 max814k/l/nmax815k/l/n reset 100k ? 1% parameter min typ max unit +12v resetthreshold at +25 c 10.57 11.00 11.45 v 340k ? 1% +5v +12v v cc pfi gnd mr pfo to p figure 10. reset valid to ground circuit figure 11. monitoring both +5v and +12v downloaded from: http:///
max814/max815/max816 negative-going v cc transients in addition to issuing a reset to the ? during power-up,power-down, and brownout conditions, the max814/ max815/max816 series is relatively immune to short duration negative-going v cc transients (glitches). the typical operating characteristics show a graph of maximum transient duration vs. reset comparatoroverdrive, for which a reset is not generated. the graph was made using a negative-going pulse applied to v cc , starting 1.5v above the actual reset threshold and ending below it by the magnitude indicated (resetcomparator overdrive). the graph indicates the typical maximum pulse width a negative-going v cc transient may have without causing a reset pulse. as the magni-tude of the transient increases (goes further below the reset threshold), the maximum allowable pulse width decreases. typically, a v cc that goes 100mv below the reset threshold and lasts 30? or less will not cause areset pulse to be issued. a 0.1? bypass capacitor mounted as close as possible to pin 2 (v cc ) provides additional transient immunity. interfacing to ps with bidirectional reset pins ?s with bidirectional reset pins, such as the motorola 68hc11 series, can cause a conflict with the reset output. if, for example, the reset output is driven high and the ? wants to pull it low, indeterminate logic lev-els may result. to correct this, connect a 4.7k ? resistor between the reset output and the ? reset i/o, as in figure 14. buffer the reset output to other system components. 1% accuracy, low-power, +3v and +5v p supervisory circuits 14 ______________________________________________________________________________________ figure 12. monitoring a negative voltagefigure 13. flow chart of wdi implementation max814max815 max816 reset r2 r1 +5v v- v cc pfi gnd mr pfo to p 100k ? 100k ? 2n3904 mr pfo +5v 0v +5v 0v v trip 0v v- v pft = 2.5v (k, l, n); 1.70v (t and max816) 5 - 2.5 = 2.5 - v trip , v trip < 0v r1 r2 begin program set low wdi set high wdi subroutine set low wdi yes no return figure 14. interfacing to ?s with bidirectional reset i/o max814max815 max816 buffered reset to other system components 4.7k ? v cc gnd reset v cc gnd reset p downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits ______________________________________________________________________________________ 15 ______________ordering information ___________________chip information part* temp. range pin-package max814 _cpa 0? to +70? 8 plastic dip max814_csa 0? to +70? 8 so max814_epa -40? to +85? 8 plastic dip max814_esa -40? to +85? 8 so max816 cpa 0? to +70? 8 plastic dip max816csa 0? to +70? 8 so max816epa -40? to +85? 8 plastic dip max816esa -40? to +85? 8 so * the max814/max815 offer a choice of reset threshold voltage. from the reset trip threshold table, select the suffix corre- sponding to the desired threshold and insert it into the blank to complete the part number. devices are available in both leaded and lead-free packaging. specify lead free by adding the + symbol at the end of the part number when ordering. max815 _cpa 0? to +70? 8 plastic dip max815_csa 0? to +70? 8 so max815_epa -40? to +85? 8 plastic dip max815_esa -40? to +85? 8 so transistor count: 744 _____________reset trip thresholds max (v) 3.06 3.00 t 4.60 4.50 n 4.75 4.65 l 4.85 4.75 k max814/max815 min (v) reset trip threshold max816 suffix adjustable revision history pages changed at rev 1: 1?, 12-16. downloaded from: http:///
max814/max815/max816 1% accuracy, low-power, +3v and +5v p supervisory circuits dim a a1a2 a3 b b1 c d1 e e1 e eaeb l min 0.0150.125 0.055 0.016 0.045 0.008 0.005 0.300 0.240 0.100 0.300 0.115 max 0.200 0.1750.080 0.022 0.065 0.012 0.080 0.325 0.310 0.4000.150 min 0.383.18 1.40 0.41 1.14 0.20 0.13 7.62 6.10 2.54 7.62 2.92 max 5.08 4.452.03 0.56 1.65 0.30 2.03 8.26 7.87 10.16 3.81 inches millimeters plastic dip plastic dual-in-line package (0.300 in.) dim dd d d d d pkg. pp p p p n min 0.3480.735 0.745 0.885 1.015 1.14 max 0.3900.765 0.765 0.915 1.045 1.265 min 8.84 18.6718.92 22.48 25.78 28.96 max 9.91 19.4319.43 23.24 26.54 32.13 inches millimeters pins 8 1416 18 20 24 c a a2 e1 d e ea eb a3 b1 b 0 - 15 a1 l d1 e 21-0043a dim a a1 b c e e h l min 0.0530.004 0.014 0.007 0.150 0.228 0.016 max 0.0690.010 0.019 0.010 0.157 0.244 0.050 min 1.350.10 0.35 0.19 3.80 5.80 0.40 max 1.750.25 0.49 0.25 4.00 6.20 1.27 inches millimeters 21-0041a narrow so small-outline package (0.150 in.) dim dd d min 0.1890.337 0.386 max 0.1970.344 0.394 min 4.808.55 9.80 max 5.008.75 10.00 inches millimeters pins 8 1416 1.27 0.050 l 0?8 h e d e a a1 c 0.101mm 0.004in. b 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. 16 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2005 maxim integrated products is a registered trademark of maxim integrated products, inc. 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 .) downloaded from: http:///

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