19-0412; Rev 0; 6/95 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits ____________________________Features _____________________Selector Table FEATURE MAX816 ✔ ♦ 4.8V, 4.7V, 4.55V, 3.03V, or Adjustable Reset Thresholds ♦ ±1% Low-Line Threshold Accuracy (MAX814) 60mV Above Reset Threshold ♦ 200ms Reset Time Delay ♦ Active-Low RESET Output Active-High RESET Output (MAX814/MAX816) ♦ 75µA Max Supply Current ♦ Guaranteed RESET Valid to VCC = 1V ♦ Manual Reset Input ♦ ±2% Power-Fail Comparator ♦ Independent Watchdog with 1.56sec Timeout (MAX815) RESET Output MAX814 ✔ RESET Output ✔ ✔ ♦ Power-Supply Glitch Immunity Manual Reset ✔ ✔ ✔ ♦ 8-Pin SO and DIP Packages Power-Fail Monitor K, L, N, T ✔ K, L, N, T ✔ Adjustable ✔ Low-Line Detector ✔ VCC Reset Voltage MAX815 ✔ ♦ ±1% Worst-Case Reset Threshold Accuracy _____________Reset Trip Thresholds ✔ Watchdog Circuit SUFFIX ________________________Applications Medical Equipment Controllers Intelligent Instruments Critical µP Power Monitoring Portable/Battery-Powered Equipment Set-Top Boxes MAX814/MAX815 RESET TRIP THRESHOLD MIN (V) MAX (V) K 4.75 4.85 L 4.65 4.75 N 4.50 4.60 T 3.00 3.06 MAX816 — Adjustable Ordering Information appears at end of data sheet. __________________________________________________________Pin Configurations TOP VIEW MR 1 8 RESET MR 1 8 WDO MR 1 8 RESET VCC 2 7 RESET VCC 2 7 RESET VCC 2 7 RESET GND 3 6 LOW LINE GND 3 6 WDI GND 3 6 RESET IN PFI 4 5 PFO PFI 4 5 PFO PFI 4 5 PFO MAX814 DIP/SO MAX815 DIP/SO MAX816 DIP/SO ________________________________________________________________ Maxim Integrated Products Call toll free 1-800-998-8800 for free samples or literature. 1 MAX814/MAX815/MAX816 _______________General Description The MAX814/MAX815/MAX816 are high-accuracy microprocessor (µP) 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 VCC down to 1V. The reset comparator is designed to ignore fast transients on VCC. Reset thresholds are available for operation with a variety of 3V and 5V supply voltages. A 75µA 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 packages. See the Selector Table below for a review of features. MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits ABSOLUTE MAXIMUM RATINGS Terminal Voltage (with respect GND) VCC ....................................................................-0.3V to +6.0V All Other Pins (Note 1)........................... -0.3V to (VCC + 0.3V) Terminal Current (PFI, RESET IN, MR)................................10mA Terminal Current (all other pins) .........................................20mA Continuous Power Dissipation (TA = +25°C) Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW SO (derate 5.88mW/°C above +70°C) .........................471mW Operating Temperature Ranges Commercial.........................................................0°C to +70°C Extended ...........................................................-40°C to +85C Storage Temperature Range .............................-65°C to +125°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, +5V Parts (MAX814/MAX815K, L, N) (VCC = 4.85V to 5.5V for MAX814K/MAX815K, VCC = 4.75V to 5.5V for MAX814L/MAX815L, VCC = 4.60V to 5.5V for MAX814N/ MAX815N, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Operating Voltage Range SYMBOL VCC Supply Current ISUPPLY Reset Threshold VRT Reset Threshold Hysteresis VRT Reset Pulse Width tRS RESET Output Voltage VOH VOL VOH CONDITIONS MIN TYP 1.0 5.5 MAX814_E, MAX815_E 1.2 5.5 MAX814_C, MAX815_C 75 MAX814_E, MAX815_E 85 MAX814K, MAX815K 4.75 4.85 MAX814L, MAX815L 4.65 4.75 MAX814N, MAX815N 4.50 4.60 0 140 MAX814 ISOURCE = 800µA 200 ISINK = 3.2mA ISOURCE = 800µA 0.4 0.4 MAX814_C/MAX815_C, VCC = 1.0V, ISINK = 50µA 0.3 VOL Watchdog Timeout Period tWD MAX815 1.12 WDI Pulse Width tWP MAX815 50 MAX814_E/MAX815_E, VCC = 1.2V, ISINK = 100µA MAX815, VCC = 5.0V WDI Input Current IWDI WDI = VCC or WDI = 0V MR to WDO High Delay tWDO MAX815 (Note 1) LOW LINE to RESET Differential Threshold ∆VLL LOW LINE Threshold VLLT 2 V µA V ms V VCC -1.5 ISINK = 3.2mA VWDI UNITS mV 250 VCC -1.5 RESET, WDO, PFO, LOW LINE Output Voltage WDI Input Threshold MAX MAX814_C, MAX815_C 0.3 2.00 sec ns Low High V 0.8 2.4 -1.0 1.0 1 µA µs MAX814_C, VCC falling 50 70 MAX814_E, VCC falling 48 73 MAX814K, VCC rising 4.93 MAX814L, VCC rising 4.83 MAX814N, VCC rising 4.68 _______________________________________________________________________________________ V mV V ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits MAX814/MAX815/MAX816 ELECTRICAL CHARACTERISTICS, +5V Parts (MAX814/MAX815K, L, N) (continued) (VCC = 4.85V to 5.5V for MAX814K/MAX815K, VCC = 4.75V to 5.5V for MAX814L/MAX815L, VCC = 4.60V to 5.5V for MAX814N/ MAX815N, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL MR Pull-Up Current IMR MR Pulse Width tMR MR Input Threshold CONDITIONS MR = 0V TYP 70 MAX UNITS 240 µA 150 V MRLO Low V MRHI High MR to RESET Out Delay tMD (Note 3) PFI Input Threshold VPFI VCC = 5.0V PFI Input Current IPFI LOW LINE, PFO, WDO Assertion Delay MIN ns 1.1 0.7 x VCC 250 V ns 2.45 2.50 2.55 V -15.00 6.0 35.00 nA (Note 2) 200 µs ELECTRICAL CHARACTERISTICS, +3V Parts (MAX814/MAX815T, MAX816) (VCC = 3.06V to 5.5V for MAX814T/MAX815T and MAX816, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Operating Voltage Range SYMBOL VCC Supply Current ISUPPLY Reset Threshold VRT RESET IN Threshold VRT RESET IN Input Current IRT Reset Threshold Hysteresis VRT Reset Pulse Width CONDITIONS RESET Output Voltage VOL VOH VOL VOH VOL RESET, WDO, PFO, LOW LINE Output Voltage VOH VOL VOL Watchdog Timeout Period tWD TYP MAX 1.0 5.5 -40°C to +85°C 1.2 5.5 0°C to +70°C 75 -40°C to +85°C 85 0°C to +70°C 3.00 3.06 -40°C to +85°C 3.00 3.08 MAX816C 1.683 1.700 1.717 MAX816E 1.678 1.700 1.722 6 35 MAX816 -15 0 tRS VOH MIN 0°C to +70°C 140 VRT(max)<VCC<3.6V; MAX814T, MAX816 ISOURCE = 500µA 4.5V<VCC<5.5V; MAX814T, MAX816 ISOURCE = 800µA VRT(max)<VCC<3.6V 4.5V<VCC<5.5V 250 0.3 V V nA ms V 0.4 0.8 x VCC ISINK = 1.2mA 0.3 VCC -1.5 ISINK = 3.2mA 0.4 TA = 0°C to +70°C, VCC = 1.0V, ISINK = 50µA V 0.3 TA = -40°C to +85°C, VCC = 1.2V, ISINK = 100µA MAX815T µA mV VCC -1.5 ISINK = 3.2mA ISOURCE = 800µA V 0.8 x VCC ISINK = 1.2mA ISOURCE = 500µA 200 UNITS 0.3 1.12 2.00 sec _______________________________________________________________________________________ 3 MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits ELECTRICAL CHARACTERISTICS, +3V Parts (MAX814/MAX815T, MAX816) (continued) (VCC = 3.06V to 5.5V for MAX814T/MAX815T and MAX816, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER WDI Pulse Width SYMBOL tWP CONDITIONS MAX815T VRT(max)<VCC<3.6V; MAX815T WDI Input Threshold VWDI VCC = 5.0V; MAX815T 100 4.5V<VCC<5.5V 50 High High WDI = VCC or 0V, MAX815T MR to WDO High Delay tWDO MAX815T (Note 1) LOW LINE Threshold VLLT MR Pull-Up Current IMR MR Pulse Width tMR MR Input Threshold 73 VCC rising VPFI PFI Input Current IPFI 3.163 VRT(max)<VCC<3.6V 70 240 4.5V<VCC<5.5V 110 370 VRT(max)<VCC<3.6V 500 4.5V<VCC<5.5V 150 1.1 0.7 x VCC 750 4.5V<VCC<5.5V, (Note 3) 250 V µA V ns 1.666 1.700 1.734 V -15.00 6.0 35.00 nA 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. 4 mV ns VRT(max)<VCC<3.6V; (Note 3) VCC = 3.3V, 5V µA µs 48 High PFI Input Threshold 1.0 1 VCC falling, MAX814TE Low V 2.4 -1.0 70 V MRHI tMD 0.8 50 MR = 0V UNITS 0.8 0.7 x VCC VCC falling, MAX814TC V MRLO MR to RESET Out Delay MAX ns Low IWDI ∆VLL TYP Low WDI Input Current LOW LINE to RESET Differential Threshold MIN VRT(max)<VCC<3.6V _______________________________________________________________________________________ ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits VCC = 3.3V 50 45 VCC = 2.5V 40 55 50 45 VCC = 3.3V 40 MAX814-03 MAX814-02 VCC = 5.0V 195 VCC = 3.3V 190 185 VCC = 5.0V 180 30 0 20 40 60 175 -60 -40 -20 80 100 0 20 40 60 80 100 TEMPERATURE (°C) TEMPERATURE (°C) RESET-COMPARATOR PROPAGATION DELAY vs. TEMPERATURE PFI THRESHOLD vs. TEMPERATURE 3V PARTS (MAX814T/MAX815T, MAX816) 1.715 1.710 25 20 1.700 1.695 1.690 1.685 0 20 40 60 2.460 2.450 -60 -40 -20 0 20 40 60 60 58 56 54 52 70 LOW-LINE TO RESET THRESHOLD (mV) 62 80 100 40 TEMPERATURE (°C) 60 -60 -40 -20 68 80 100 0 20 40 60 80 100 TEMPERATURE (°C) LOW-LINE COMPARATOR PROPAGATION DELAY vs. TEMPERATURE 66 64 62 60 58 56 54 60 100mV OVERDRIVE (VLLT - VCC) 50 40 30 20 10 52 50 50 20 2.480 LOW-LINE TO RESET THRESHOLD vs. TEMPERATURE (VCC FALLING) 64 0 2.490 TEMPERATURE (°C) 66 -60 -40 -20 2.500 2.470 80 100 MAX814-10 70 2.510 1.675 LOW-LINE TO RESET THRESHOLD vs. TEMPERATURE (VCC RISING) 68 2.520 1.680 TEMPERATURE (°C) 80 100 2.550 PROPAGATION DELAY (µs) -60 -40 -20 60 2.530 1.670 15 40 2.540 PFI THRESHOLD (V) 30 20 PFI THRESHOLD vs. TEMPERATURE 5V PARTS (MAX814/MAX815K, L, N) 1.705 PFI THRESHOLD (V) 35 0 TEMPERATURE (°C) MAX814-05 100mV OVERDRIVE (VRT - VCC) MAX814-04 40 -60 -40 -20 MAX814-06 -60 -40 -20 PROPAGATION DELAY (µs) 60 35 30 LOW-LINE TO RESET THRESHOLD (mV) 200 MAX814-11 35 RESET TIMEOUT PERIOD vs. TEMPERATURE MAX814-09 VCC SUPPLY CURRENT (µA) 60 55 70 65 VCC SUPPLY CURRENT (µA) VCC = 5.0V MAX814-01 70 65 VCC SUPPLY CURRENT vs. TEMPERATURE 5V PARTS (MAX814/MAX815K, L, N) RESET TIMEOUT PERIOD (ms) VCC SUPPLY CURRENT vs. TEMPERATURE 3V PARTS (MAX814T/MAX815T, MAX816) 0 -60 -40 -20 0 20 40 TEMPERATURE (°C) 60 80 100 -60 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX814/MAX815/MAX816 __________________________________________Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) ____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) RESET THRESHOLD DEVIATION vs. TEMPERATURE 5V PARTS (MAX814/MAX815K, L, N) RESET THRESHOLD DEVIATION vs. TEMPERATURE 3V PARTS (MAX814T/MAX815T, MAX816) RESET THRESHOLD DEVIATION (mV) 1 0 -1 -2 MAX814-08 1 MAX814-07 RESET THRESHOLD DEVIATION (mV) 2 0 -1 -2 -3 -60 -40 -20 0 40 20 60 -60 -40 -20 80 100 100 80 RESET OCCURS ABOVE CURVE 60 40 20 80 100 MAX814-13 120 100 RESET OCCURS ABOVE CURVE 80 60 40 20 0 0 10 100 1000 RESET COMPARATOR OVERDRIVE (mV), VRT - VCC 6 60 140 MAXIMUM TRANSIENT DURATION (µs) MAX814-12 120 40 20 MAXIMUM TRANSIENT DURATION vs. RESET COMPARATOR OVERDRIVE 5V PARTS (MAX814/MAX815K, L, N) MAXIMUM TRANSIENT DURATION vs. RESET COMPARATOR OVERDRIVE 3V PARTS (MAX814T/MAX815T, MAX816) 140 0 TEMPERATURE (°C) TEMPERATURE (°C) MAXIMUM TRANSIENT DURATION (µs) MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits 10 100 1000 RESET COMPARATOR OVERDRIVE (mV), VRT - VCC _______________________________________________________________________________________ ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits PIN MAX814 MAX815 MAX816 1 1 1 2 2 — NAME FUNCTION MR Manual-Reset Input. Triggers a reset when pulled below 1.10V. This activelow input has an internal 150µA pull-up current to VCC, and can be driven with CMOS logic or shorted to GND with a switch or transistor. VCC Positive Power-Supply Input. When VCC is below the reset threshold voltage*, RESET is low, and remains low for a minimum of 140ms after it rises above the threshold. Positive Power-Supply Input. On the MAX816, RESET is controlled by RESET IN, not VCC. — — 2 3 3 3 GND 4 4 4 PFI Power-Fail Input. The PFI threshold voltage is 1.70V on the MAX816 and on MAX814/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 VCC 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 1.56sec, the watchdog timer times out, and WDO goes low. The timer is reset to zero on each WDI transition. 6 — — Low-Line Output. Normally high, LOW LINE goes low when VCC falls 60mV LOW LINE above the reset threshold. It returns high as soon as VCC rises above the low-line threshold. — — 6 RESET IN 7 7 7 RESET — 8 — WDO 8 — 8 RESET Ground Reset Comparator Input. Reference is 1.70V. When RESET IN is below 1.70V, RESET is low, and remains low for a minimum of 140ms after it rises above the reference. Reset Output. Normally high, active low. Controlled by MR and reset comparator. Watchdog Output. Normally high, WDO goes low whenever the VCC reset threshold comparator input voltage is low or when the watchdog timer times out. There is no appreciable delay going either direction when the VCC threshold comparator toggles. Reset Output. Active high. The inverse of RESET. *Reset Threshold Voltage is determined by part number suffix: K = 4.80V, L = 4.70V, N = 4.55V, T = 3.03V. _______________Detailed Description The MAX814/MAX815/MAX816 are high-accuracy, lowpower microprocessor (µP) supervisory circuits. They have µP-reset, watchdog-timer, and power-fail functions. 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. _______________________________________________________________________________________ 7 MAX814/MAX815/MAX816 ______________________________________________________________Pin Description MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits VCC +3.3V +5V +12V SUPPLY VCC VCC µP MAX814 MAX814K/L/N RESET RESET PFI PFI MR NMI LOW LINE RESET MANUAL RESET INPUT RESET INTERRUPT PFO MR RESET PFO GND GND Figure 1a. Typical Application for Dual +3.3V and +5V Systems Figure 1b. MAX814 Typical Application VRT VCC VCC +12V SUPPLY +12V SUPPLY VCC VCC µP R1 µP MAX816 MAX815 RESET RESET RESET IN PFI WDI MANUAL RESET INPUT WDO MR PFO GND PFI I/O LINE RESET NMI R2 INTERRUPT PFO MANUAL RESET INPUT RESET INTERRUPT RESET MR GND ON POWER-UP, RESET IS LOW UNTIL VCC > 2.8V. Figure 2. MAX815 Typical Application 8 Figure 3. MAX816 Typical Application _______________________________________________________________________________________ ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits 8 RESET 150µA 1 MR RESET GENERATOR 7 MAX814/MAX815/MAX816 MAX814 MAX815 6 WDI WATCHDOG TRANSITION DETECTOR WATCHDOG TIMER 8 WDO RESET TIMEBASE 2 1 VCC MR 150µA 7 RESET GENERATOR RESET 60mV 2 VCC 6 LOW LINE RESET THRESHOLD 4 PFI RESET THRESHOLD 4 PFI 5 5 PFO POWER- FAIL THRESHOLD PFO POWER-FAIL THRESHOLD 3 GND 3 GND Figure 4. MAX814 Block Diagram Figure 5. MAX815 Block Diagram Reset Output 2 VCC MAX816 150µA 1 RESET GENERATOR MR 8 RESET 7 RESET 6 RESET IN 1.70V 4 PFI 5 PFO A µP’s reset input starts the µP in a known state. Whenever the µP 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 VCC reaches 1V, RESET is a guaranteed logic low of 0.4V or less. As VCC rises, RESET stays low. As VCC rises above the reset threshold, an internal timer releases RESET after 200ms. RESET also pulses low whenever VCC dips below the reset threshold (i.e., brownout condition). If brownout occurs in the middle 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 VCC falls below the reset threshold, RESET stays low and is guaranteed to be less than 0.3V until VCC drops below 1V. The MAX814 and MAX816 also offer active-high RESET outputs. They are the inverse of the RESET outputs. 1.70V 3 GND Figure 6. MAX816 Block Diagram _______________________________________________________________________________________ 9 MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits Reset Threshold The MAX814/MAX815 have fixed, factory-set reset thresholds, signified by the first suffix letter in the part number (see Figure 7 for more information on reset ranges). 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 ±4% or better power-supply tolerance design that must meet worst-case system parameters over time, temperature, line, and load variations. Typically, the reset threshold (VRT) is greater than or equal to the minimum IC operating voltage (VICMIN). The “K” series 1%-tolerance reset threshold allows a larger range of power-supply tolerance. System ICs 4.5 4.75 5.0 5.25 that have a tight operating supply range, like the 386/486 µPs, 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 ±5% power-supply tolerance. Typically, the reset threshold is less than or equal to the minimum powersupply voltage, allowing system operation over the complete power-supply range. A reset is initiated at 4.75V maximum. The 1% “L” 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 ±10% IC system. Typically, the reset threshold 4.5 5.5V POWER-SUPPLY RANGE (5.05V ±2%) POWER-SUPPLY RANGE (5V ±5%) IC OPERATING RANGE IC OPERATING RANGE RESET LIMITS “K” (4.8V ±1%) RESET LIMITS “L” (4.7V ±1%) 4.85 Figure 7a. K Suffix Design Range 4.75 5.0 5.25 5.5V 4.65 5.5V 3.0 3.3 3.6V POWER-SUPPLY RANGE (3.33V ±0.26%) IC OPERATING RANGE IC OPERATING RANGE RESET LIMITS “T” (3.03V ±1%) 4.60 VRT > VIC(MIN) DESIGNED TO MEET WORSTCASE DESIGN PARAMETERS. Figure 7c. N Suffix Design Range 10 5.25 Figure 7b. L Suffix Design Range POWER-SUPPLY RANGE (5V ±5%) RESET LIMITS “N” (4.55V ±1%) 5.0 VRT < VPS(MIN) DESIGNED TO ALLOW OPERATION OVER THE FULL POWER-SUPPLY RANGE. VRT > VIC(MIN) DESIGNED TO MEET WORSTCASE DESIGN PARAMETERS. 4.5 4.75 3.06 VRT > VIC(MIN) DESIGNED TO MEET WORSTCASE DESIGN PARAMETERS. Figure 7d. T Suffix Design Range ______________________________________________________________________________________ ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits VRT = ( ) VRIT × R1 + R2 R2 where VRT = the desired reset threshold, VRIT is the RESET IN threshold (1.700V), R1 is the resistor connected 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 (100kΩ, for example) and calculate R1 based on the desired reset threshold, using the following formula: [( ) ] R1 = R2 × VRT / VRIT − 1 The MAX816 can achieve ±1.2% accuracy with 0.1% resistors. Watchdog Timer (MAX815) The watchdog circuit monitors the µP’s activity. If the µP does not toggle the watchdog input (WDI) within the watchdog timeout period (tWP), WDO goes low (Figure 8). WDO also goes low during reset conditions. Whenever VCC is below the reset threshold, WDO stays low; however, unlike RESET, WDO does not have a minimum pulse width. As soon as VCC rises above the reset threshold, WDO goes high with no delay (Figure 9). Typically, WDO is connected to the non-maskable interrupt (NMI) of a µP. When VCC drops below the reset threshold, WDO goes low whether or not the watchdog timer has timed out (Figure 9). This would normally trigger an NMI interrupt, but RESET goes low simultaneously 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 indicate when power is failing. This warning provides time for the µP to store vital data and take care of any additional “housekeeping” before the power supply gets too far out of tolerance for the µP to operate reliably. Power-Fail Comparator The power-fail comparator is intended as an undervoltage 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 VPFI just before the monitored voltage drops out. Use PFO to interrupt the µP, 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 powerfail reference, PFO sinks at least 1.2mA to GND; otherwise it sources at least 300µA from VCC. The reference is 2.50V in the MAX814/MAX815 with K, L, N suffixes, or 1.70V with the T suffix. It is also 1.70V in the MAX816. LOW LINE Output (MAX814) The low-line detector is a separate comparator that monitors VCC with a typical threshold voltage of 60mV above the normal reset threshold, with 2mV of hysteresis (Figure 9). If VCC rises faster than 10µs/V, insert a 100pF capacitor from LOW LINE to GND to ensure proper start-up. For normal operation (VCC above the reset threshold), LOW LINE is pulled to VCC. Use LOW LINE to provide an NMI to the µP when power begins to fall. In most battery-operated portable systems, reserve energy 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 VCC fall time— such as when the main battery is disconnected or a high-side switch is opened during operation—use capacitance on the VCC line to provide time to execute the shutdown routine. First, calculate the worst-case time required for the system to perform its shutdown routine. Then use the worst-case shutdown time (tSHDN), worst-case load current (ILOAD), and minimum low-line to reset threshold (VLR) to calculate the amount of capacitance required to allow the shutdown routine to complete before reset is asserted. I ×t CHOLD = LOAD SHDN VLR ______________________________________________________________________________________ 11 MAX814/MAX815/MAX816 (VRT) is greater than or equal to the minimum IC operating voltages (VICMIN). The 1% “L” series allows the use of a 5V ±5% power supply, and guarantees system operation 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 ±0.26V) with a 7.8% or better power-supply tolerance. Typically, the reset threshold (VRT) is greater than or equal to the minimum IC operating voltages (VICMIN). The MAX816 has an adjustable reset threshold, set with an external resistive divider (Figure 3). The voltage on the RESET IN pin is monitored, not the voltage on VCC. The RESET IN threshold is 1.700V, and has very high impedance and 35nA maximum leakage. Calculate the trip point, VRT, as follows: MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits tWP tWD tWD tWDO tWD VCC ∆VLL 60mV VRT VLLT VRT VCC WDI 0V tRS tRS MR RESET VCC WDO 0V RESET RESET VCC VCC 0V MR VCC 0 tMD 0V tMR VCC WDO (MAX815) 0 Figure 8. MAX815 Watchdog Timing VCC LOW LINE (MAX814) 0 where CHOLD is the capacitance (in Farads), ILOAD is the current being drained from the capacitor (in Amperes), and VLR is the low-line to reset threshold difference (in Volts). Figure 9. Timing Diagram __________Applications Information Manual Reset Low-Voltage Operation Many µP-based products require manual-reset capability, 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 tRS (200ms) after MR returns high. This input has an internal pull-up resistor, so it can be left open if not used. MR can be driven with TTL/CMOS-logic levels or with open-drain/collector outputs. Connect a normally open momentary switch from MR to GND to create a manual-reset function; external debounce 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 environment, connect a 0.1µF capacitor to ground to provide additional noise immunity. The LOW LINE, PFO, and WDO outputs will be locked to logic low when the power supply drops below the lockout threshold (typically 1V below the reset threshold). 12 Ensuring a Valid RESET Output Down to VCC = 0V When VCC falls below 1V, the RESET output no longer sinks current, but becomes an open circuit. Highimpedance CMOS-logic inputs can drift to undetermined voltages if left undriven. If a pull-down 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 100kΩ—large enough not to load RESET, and small enough to pull RESET to ground. ______________________________________________________________________________________ ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits +5V VCC 340k 1% MAX814 MAX815 MAX816 MAX814K/L/N MR MAX815K/L/N 100k 1% RESET TO µP RESET PFI PFO GND R1 PARAMETER MIN TYP MAX +12V Reset 10.57 11.00 11.45 Threshold at +25°C UNIT V Figure 10. RESET Valid to Ground Circuit Figure 11. Monitoring Both +5V and +12V Monitoring Voltages Other than VCC Monitor voltages other than the V CC by connecting a voltage divider to PFI and adjusting the ratio appropriately. If required, add hysteresis by connecting a resistor (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’s sensitivity to high-frequency noise on the line being monitored. RESET can be asserted on other voltages in addition to the +5V VCC line. Connect PFO to MR to initiate a reset when PFI drops below 2.50V (K, L, N suffix) or 1.70V (T suffix or 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. A way to help the watchdog timer keep closer tabs on software 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 program should hang in any subroutine, the I/O is continually set high and the watchdog timer is allowed to time out, causing a reset to be issued. Watchdog Software Considerations Monitoring a Negative Voltage The power-fail comparator can also monitor a negative supply 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 resistors 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’s accuracy depends on the PFI threshold tolerance, the VCC line, and the resistor. ______________________________________________________________________________________ 13 MAX814/MAX815/MAX816 +12V MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits +5V BUFFERED RESET TO OTHER SYSTEM COMPONENTS VCC 100k MR R1 MAX814 MAX815 PFO PFI MAX816 R2 VCC 100k 2N3904 TO µP RESET GND VCC MAX814 MAX815 MAX816 µP 4.7k RESET RESET GND V- GND +5V MR 0V +5V PFO Figure 14. Interfacing to µPs with Bidirectional Reset I/O 0V V- VTRIP 0V 5 - 2.5 2.5 - VTRIP , VTRIP < 0V = R1 R2 VPFT = 2.5V (K, L, N); 1.70V (T AND MAX816) Figure 12. Monitoring a Negative Voltage BEGIN PROGRAM SET LOW WDI SET HIGH WDI Negative-Going VCC Transients In addition to issuing a reset to the µP during power-up, power-down, and brownout conditions, the MAX814/ MAX815/MAX816 series is relatively immune to short duration negative-going VCC transients (glitches). The Typical Operating Characteristics show a graph of Maximum Transient Duration vs. Reset Comparator Overdrive, for which a reset is not generated. The graph was made using a negative-going pulse applied to VCC, starting 1.5V above the actual reset threshold and ending below it by the magnitude indicated (reset comparator overdrive). The graph indicates the typical maximum pulse width a negative-going VCC transient may have without causing a reset pulse. As the magnitude of the transient increases (goes further below the reset threshold), the maximum allowable pulse width decreases. Typically, a VCC that goes 100mV below the reset threshold and lasts 30µs or less will not cause a reset pulse to be issued. A 0.1µF bypass capacitor mounted as close as possible to pin 2 (VCC) provides additional transient immunity. SUBROUTINE Interfacing to µPs with Bidirectional Reset Pins SET LOW WDI YES RETURN NO µPs 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 µP wants to pull it low, indeterminate logic levels may result. To correct this, connect a 4.7kΩ resistor between the RESET output and the µP reset I/O, as in Figure 14. Buffer the RESET output to other system components. Figure 13. Flow Chart of WDI Implementation 14 ______________________________________________________________________________________ ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits PART* TEMP. RANGE PIN-PACKAGE 0°C to +70°C 8 Plastic DIP MAX814_CSA 0°C to +70°C 8 SO MAX814_EPA -40°C to +85°C MAX814_ESA -40°C to +85°C MAX815_CPA 0°C to +70°C 8 Plastic DIP MAX815_CSA 0°C to +70°C 8 SO MAX815_EPA -40°C to +85°C 8 Plastic DIP MAX815_ESA -40°C to +85°C 8 SO MAX816CPA 0°C to +70°C 8 Plastic DIP MAX816CSA 0°C to +70°C 8 SO MAX816EPA -40°C to +85°C 8 Plastic DIP MAX816ESA -40°C to +85°C 8 SO MAX814_CPA ___________________Chip Information TRANSISTOR COUNT: 744 8 Plastic DIP 8 SO * The MAX814/MAX815 offer a choice of reset threshold voltage. From the Reset Trip Threshold table, select the suffix corresponding to the desired threshold and insert it into the blank to complete the part number. _____________Reset Trip Thresholds MAX814/MAX815 SUFFIX RESET TRIP THRESHOLD MIN (V) MAX (V) K 4.75 4.85 L 4.65 4.75 N 4.50 4.60 T 3.00 3.06 — — MAX816 Adjustable ______________________________________________________________________________________ 15 MAX814/MAX815/MAX816 ______________Ordering Information MAX814/MAX815/MAX816 ±1% Accuracy, Low-Power, +3V and +5V µP Supervisory Circuits ________________________________________________________Package Information D E DIM E1 A A1 A2 A3 B B1 C D1 E E1 e eA eB L A3 A A2 L A1 0° - 15° C e B1 eA B eB D1 Plastic DIP PLASTIC DUAL-IN-LINE PACKAGE (0.300 in.) INCHES MAX MIN 0.200 – – 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.045 0.012 0.008 0.080 0.005 0.325 0.300 0.310 0.240 – 0.100 – 0.300 0.400 – 0.150 0.115 PKG. DIM PINS P P P P P N D D D D D D 8 14 16 18 20 24 INCHES MIN MAX 0.348 0.390 0.735 0.765 0.745 0.765 0.885 0.915 1.015 1.045 1.14 1.265 MILLIMETERS MIN MAX – 5.08 0.38 – 3.18 4.45 1.40 2.03 0.41 0.56 1.14 1.65 0.20 0.30 0.13 2.03 7.62 8.26 6.10 7.87 2.54 – 7.62 – – 10.16 2.92 3.81 MILLIMETERS MIN MAX 8.84 9.91 18.67 19.43 18.92 19.43 22.48 23.24 25.78 26.54 28.96 32.13 21-0043A DIM D 0°-8° A 0.101mm 0.004in. e B A1 E C L Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.) H A A1 B C E e H L INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016 DIM PINS D D D 8 14 16 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27 INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00 21-0041A 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 © 1995 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.