_______________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 com-
parator is designed to ignore fast transients on VCC.
Reset thresholds are available for operation with a vari-
ety 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 pack-
ages. See the Selector Table for a review of features.
________________________Applications
Medical Equipment
Controllers
Intelligent Instruments
Critical µP 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 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)
♦Power-Supply Glitch Immunity
♦8-Pin SO and DIP Packages
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
________________________________________________________________ Maxim Integrated Products 1
LOW LINE
GND
PFO
PFI
1
2
8
7
RESET
RESET
VCC
MR
MAX814
DIP/SO
TOP VIEW
3
4
6
5
WDI
GND
PFO
PFI
1
2
8
7
WDO
RESET
VCC
MR
MAX815
DIP/SO
3
4
6
5
RESET IN
GND
PFO
PFI
1
2
8
7
RESET
RESET
VCC
MR
MAX816
DIP/SO
3
4
6
5
__________________________________________________________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 ✔ ✔ ✔
VCC 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’s website at www.maxim-ic.com.
PART* TEMP RANGE PIN-PACKAGE
MAX814_CPA 0°C to +70°C 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.
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
2_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
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.)
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.
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
MAX814K, VCC rising
MAX814_E, VCC falling
MAX814_C, VCC falling
MAX814_C, MAX815_C
MAX815 (Note 1)
WDI = VCC or WDI = 0V
MAX814_E, MAX815_E
MAX815, VCC = 5.0V
MAX814_C, MAX815_C
MAX815
MAX815
MAX814
MAX814_E/MAX815_E, VCC = 1.2V, ISINK = 100µA
MAX814_C/MAX815_C, VCC = 1.0V, ISINK = 50µA
MAX814_E, MAX815_E
MAX814K, MAX815K
MAX814L, MAX815L
ISINK = 3.2mA
MAX814N, MAX815N
ISOURCE = 800µA
CONDITIONS
4.93
mV
48 73
∆VLL
LOW LINE to RESET Differential
Threshold
50 70
µs1tWDO
MR to WDO High Delay
µA-1.0 1.0IWDI
WDI Input Current
V
2.4
VWDI
WDI Input Threshold 0.8
ns50tWP
WDI Pulse Width
s1.12 2.00tWD
Watchdog Timeout Period
V
0.3
RESET, WDO, PFO, LOW LINE
Output Voltage 0.3
0.4
VOL
VCC -1.5VOH
75
V
1.2 5.5
VCC
1.0 5.5
Operating Voltage Range
V
0.4VOL
RESET Output Voltage VCC -1.5VOH
ms140 200 250tRS
Reset Pulse Width
mV0Reset Threshold Hysteresis
µA
85
ISUPPLY
Supply Current
4.75 4.85
4.65 4.75 V
4.50 4.60
VRT
Reset Threshold
UNITSMIN TYP MAXSYMBOLPARAMETER
MAX814L, VCC rising 4.83
VLLT
MAX814N, VCC rising 4.68
VLOW LINE Threshold
ISOURCE = 800µA
ISINK = 3.2mA
Low
High
ISINK = 3.2mA
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
_______________________________________________________________________________________ 3
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.)
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.)
0°C to +70°C
-40°C to +85°C
0°C to +70°C
MAX815T
TA= -40°C to +85°C, VCC = 1.2V, ISINK = 100µA
TA= 0°C to +70°C, VCC = 1.0V, ISINK = 50µA
MAX816
-40°C to +85°C
0°C to +70°C
-40°C to +85°C
CONDITIONS
ISOURCE = 500µA
s1.12 2.00tWD
Watchdog Timeout Period
0.3
ISINK = 1.2mA
0.3
0.3
VOL
0.8 x VCC
VOH
75
V
1.2 5.5
VCC
1.0 5.5
Operating Voltage Range
0.3VOL
0.8 x VCC
VOH
ms140 200 250tRS
Reset Pulse Width
nA-15 +6 +35IRT
RESET IN Input Current
µA
85
ISUPPLY
Supply Current
3.00 3.06
3.00 3.08 VVRT
Reset Threshold
UNITSMIN TYP MAXSYMBOLPARAMETER
MAX816C
MAX816E
1.683 1.700 1.717
1.678 1.700 1.722 VVRIT
RESET IN Threshold
mV0Reset Threshold Hysteresis
ISOURCE = 800µA V
0.4VOL
RESET Output Voltage VCC -1.5VOH
ISOURCE = 500µA
ISINK = 1.2mAVOL
RESET, WDO, PFO, LOW LINE
Output Voltage V
0.4
VCC -1.5VOH ISOURCE = 800µA
ISINK = 3.2mAVOL
CONDITIONS
MR Input Threshold V MRHI High 0.7 x VCC
V
MR to RESET Out Delay tMD (Note 3) 250 ns
PFI Input Threshold VPFI VCC = 5.0V 2.45 2.50 2.55 V
PFI Input Current IPFI
UNITSMIN TYP MAXSYMBOLPARAMETER
MR Pull-Up Current IMR MR = 0V 70 240 µA
MR Pulse Width tMR 150 ns
VMRLO Low 1.1
-15.00 +6.0 +35.00 nA
LOW LINE, PFO, WDO
Assertion Delay (Note 2) 200 µs
VRT(max) < VCC <3.6V;
MAX814T, MAX816
4.5V < VCC < 5.5V;
MAX814T, MAX816
VRT(max) < VCC < 3.6V
4.5V < VCC < 5.5V
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
4_______________________________________________________________________________________
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.)
VCC rising
VCC falling, MAX814TE
VCC falling, MAX814TC
MAX815T (Note 1)
WDI = VCC or 0V, MAX815T
VRT(max) < VCC <
3.6V; MAX815T
MAX815T
Low
CONDITIONS
3.163
MR Input Threshold
mV
48 73
∆VLL
LOW LINE to RESET Differential
Threshold
50 70
VMRHI High
µs1tWDO
MR to WDO High Delay
µA-1.0 +1.0IWDI
WDI Input Current
V
0.7 x VCC
0.8
0.7 x VCC
V
100
tWP
WDI Pulse Width
High
MR to RESET Out Delay
(Note 3) tMD
VRT(max) < VCC < 3.6V 750
PFI Input Threshold VPFI VCC = 3.3V, 5V 1.666 1.700 1.734 V
PFI Input Current IPFI
UNITSMIN TYP MAXSYMBOLPARAMETER
VLLT V
MR Pullup Current IMR MR = 0V 70 240 µA
MR Pulse Width tMR
VRT(max) < VCC < 3.6V 500 ns
V MRLO Low 1.1
-15.00 +6.0 +35.00 nA
LOW LINE Threshold
504.5V < VCC < 5.5V
VRT(max) < VCC < 3.6V
VCC = 5.0V; MAX815T Low
2.4
VWDI
WDI Input Threshold 0.8
High
110 370
4.5V < VCC < 5.5V 150
4.5V < VCC < 5.5V 250 ns
VRT(max) < VCC< 3.6V
4.5V < VCC < 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
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
_______________________________________________________________________________________ 5
70
30
-60 -40 -20 40 100
VCC SUPPLY CURRENT vs. TEMPERATURE
3V PARTS (MAX814T/MAX815T, MAX816)
45
40
35
65
MAX814-01
TEMPERATURE (°C)
VCC SUPPLY CURRENT (µA)
020 6080
60
55
50
VCC = 5.0V
VCC = 3.3V
VCC = 2.5V
70
30
-60 -40 -20 40 100
VCC SUPPLY CURRENT vs. TEMPERATURE
5V PARTS (MAX814/MAX815K, L, N)
45
40
35
65
MAX814-02
TEMPERATURE (°C)
VCC SUPPLY CURRENT (µA)
020 6080
60
55
50
VCC = 5.0V
VCC = 3.3V
200
175
-60 -40 -20 40 100
RESET TIMEOUT PERIOD
vs. TEMPERATURE
185
180
MAX814-03
TEMPERATURE (°C)
RESET TIMEOUT PERIOD (ms)
020 6080
195
190
VCC = 5.0V
VCC = 3.3V
40
15
-60 -40 -20 40 100
RESET-COMPARATOR PROPAGATION
DELAY vs. TEMPERATURE
25
20
MAX814-04
TEMPERATURE (°C)
PROPAGATION DELAY (µs)
020 6080
35
30
100mV OVERDRIVE
(VRT - VCC)
1.715
1.670
-60 -40 -20 40 100
PFI THRESHOLD vs. TEMPERATURE
3V PARTS (MAX814T/MAX815T, MAX816)
1.685
1.680
1.675
1.710
MAX814-05
TEMPERATURE (°C)
PFI THRESHOLD (V)
020 6080
1.705
1.700
1.695
1.690
2.550
2.450
-60 -40 -20 40 100
PFI THRESHOLD vs. TEMPERATURE
5V PARTS (MAX814/MAX815K, L, N)
2.490
2.470
2.480
2.460
2.540
MAX814-06
TEMPERATURE (°C)
PFI THRESHOLD (V)
020 6080
2.530
2.520
2.500
2.510
__________________________________________Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
60
0
-60 -40 -20 40 100
LOW-LINE COMPARATOR
PROPAGATION DELAY vs. TEMPERATURE
30
20
10
MAX814-11
TEMPERATURE (°C)
PROPAGATION DELAY (µs)
020 6080
50
40
100mV OVERDRIVE
(VLLT - VCC)
70
50
-60 -40 -20 40 100
LOW-LINE TO RESET THRESHOLD
vs. TEMPERATURE (VCC RISING)
58
54
56
52
68
MAX814-10
TEMPERATURE (°C)
LOW-LINE TO RESET THRESHOLD (mV)
020 6080
66
64
60
62
70
50
-60 -40 -20 40 100
LOW-LINE TO RESET THRESHOLD
vs. TEMPERATURE (VCC FALLING)
58
54
56
52
68
MAX814-09
TEMPERATURE (°C)
LOW-LINE TO RESET THRESHOLD (mV)
020 6080
66
64
60
62
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP 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
1
0
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), VRT - VCC
MAXIMUM TRANSIENT DURATION (µs)
80
60
40
100
120
RESET OCCURS
ABOVE CURVE
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, 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
1
0
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
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
_______________________________________________________________________________________ 7
_______________Detailed Description
The MAX814/MAX815/MAX816 are high-accuracy, low-
power microprocessor (µP) supervisory circuits. They
have µP-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 150µA pullup current to VCC, and can be driven
with CMOS logic or shorted to GND with a switch or transistor.
2 2 —
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.
— — 2
VCC
Positive Power-Supply Input. On the MAX816, RESET is controlled by
RESET IN, not VCC.
3 3 3 GND Ground
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 1.56s, the
watchdog 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 VCC falls 60mV
above the reset threshold. It returns high as soon as VCC rises above the
low-line threshold.
— — 6 RESET IN
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.
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 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 Threshold Voltage is determined by part number suffix: K = 4.80V, L = 4.70V, N = 4.55V, T = 3.03V.
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
8_______________________________________________________________________________________
MAX814K/L/N
VCC
+5V
+3.3V
PFI
MR
PFO
GND
RESET
RESET
MAX814
VCC
VCC
+12V SUPPLY
GND
MANUAL
RESET
INPUT
PFI
MR
RESET
µP
RESET
NMI
INTERRUPT
LOW LINE
PFO
RESET
MAX816
VCC
VCC
VRT
+12V SUPPLY
GND
MANUAL
RESET
INPUT
PFI
ON POWER-UP, RESET IS LOW UNTIL VCC > 2.8V.
R1
R2
MR
RESET IN
µP
RESET
INTERRUPT
RESET
PFO
RESET
MAX815
VCC
VCC
+12V SUPPLY
GND
MANUAL
RESET
INPUT
PFI
MR
RESET
µP
RESET
I/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
Reset Output
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 guar-
anteed 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 thresh-
old (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.
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
_______________________________________________________________________________________ 9
MAX814
RESET
GENERATOR
RESET THRESHOLD
POWER- FAIL THRESHOLD
60mV
1150µA
8
7
6
5
MR
RESET
RESET
LOW LINE
PFO
VCC
PFI
2
4
GND
3
MAX815
TIMEBASE
RESET THRESHOLD
POWER-FAIL THRESHOLD
150µA
68
7
5
MR
VCC
WDO
RESET
PFO
WDI
PFI
2
1
4
GND
3
RESET
GENERATOR
WATCHDOG
TIMER
WATCHDOG
TRANSITION
DETECTOR
MAX816
RESET
GENERATOR
1.70V
1.70V
1
2
150µA
8
7
5
MR
VCC
RESET
RESET
PFO
RESET IN
PFI
6
4
GND
3
Figure 4. MAX814 Block Diagram Figure 5. MAX815 Block Diagram
Figure 6. MAX816 Block Diagram
MAX814/MAX815/MAX816
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 tol-
erance design that must meet worst-case system para-
meters 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
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 power-
supply 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
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
10 ______________________________________________________________________________________
;;
;;
;;;
4.5 4.75
4.85
5.0 5.25 5.5V
IC OPERATING RANGE
RESET LIMITS “K”
(4.8V ±1%)
POWER-SUPPLY RANGE
(5.05V ±2%)
VRT > VIC(MIN)
DESIGNED TO MEET WORST-
CASE DESIGN PARAMETERS.
;
;;;
;;;;;
4.5 4.75
4.65
5.0 5.25 5.5V
IC OPERATING RANGE
RESET LIMITS “L”
(4.7V ±1%)
POWER-SUPPLY RANGE
(5V ±5%)
VRT < VPS(MIN)
DESIGNED TO ALLOW OPERATION
OVER THE FULL POWER-SUPPLY RANGE.
;;
;;;
;;;;;
;;;;;
4.5 4.75
4.60
5.0 5.25 5.5V
IC OPERATING RANGE
RESET LIMITS “N”
(4.55V ±1%)
POWER-SUPPLY RANGE
(5V ±5%)
VRT > VIC(MIN)
DESIGNED TO MEET WORST-
CASE DESIGN PARAMETERS.
;;
;;
;;;
3.0
3.06
3.3 3.6V
IC OPERATING RANGE
RESET LIMITS “T”
(3.03V ±1%)
POWER-SUPPLY RANGE
(3.33V ±0.26%)
VRT > VIC(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
(VRT) is greater than or equal to the minimum IC operat-
ing 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 tol-
erance. 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:
where VRT = the desired reset threshold, VRIT is the
RESET IN threshold (1.700V), R1 is the resistor con-
nected between VRT 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:
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 mini-
mum 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 inter-
rupt (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 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 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 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 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 power-
fail reference, PFO sinks at least 1.2mA to GND; other-
wise 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 hystere-
sis (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.
CIt
V
HOLD LOAD SHDN
LR
=×
RR V V
RT RIT
12 1=×
()
−
[]
/
VVRR
R
RT
RIT
=×+
()
12
2
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
______________________________________________________________________________________ 11
MAX814/MAX815/MAX816
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 dif-
ference (in Volts).
Manual Reset
Many µP-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 tRS (200ms) after MR returns high. This input
has an internal pullup resistor, so it can be left open if
not 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; 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 envi-
ronment, connect a 0.1µF 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 VCC = 0V
When VCC 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 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
12 ______________________________________________________________________________________
Figure 9. Timing Diagram
∆VLL
VCC
0
0
0
VCC
VCC
VCC
VLLT
60mV
VRT
VRT
tRS
tMR
tMD
tRS
LOW LINE
(MAX814)
WDO
(MAX815)
MR
RESET
Figure 8. MAX815 Watchdog Timing
VCC
0V
VCC
0V
VCC
0V
VCC
0V
MR
WDI
WDO
RESET
RESET
tWD
tWP tWD tWD
tWDO
Monitoring Voltages Other than VCC
Monitor voltages other than the VCC 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’s sensitivity to high-fre-
quency 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.
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 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’s accuracy depends on the
PFI threshold tolerance, the VCC line, and the resistor.
Watchdog Software Considerations
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 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
µP Supervisory Circuits
______________________________________________________________________________________ 13
MAX814
MAX815
MAX816
RESET
R1
MAX814K/L/N
MAX815K/L/N
RESET
100kΩ
1%
PARAMETER MIN TYP MAX UNIT
+12V Reset
Threshold at +25°C10.57 11.00 11.45 V
340kΩ
1%
+5V
+12V
VCC
PFI
GND
MR
PFO
TO µP
Figure 10. RESET Valid to Ground Circuit Figure 11. Monitoring Both +5V and +12V
MAX814/MAX815/MAX816
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 magni-
tude 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.
Interfacing to µPs
with Bidirectional Reset Pins
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 lev-
els may result. To correct this, connect a 4.7kΩresistor
between the RESET output and the µP reset I/O, as in
Figure 14. Buffer the RESET output to other system
components.
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
14 ______________________________________________________________________________________
Figure 12. Monitoring a Negative Voltage
Figure 13. Flow Chart of WDI Implementation
MAX814
MAX815
MAX816
RESET
R2
R1
+5V
V-
VCC
PFI
GND
MR
PFO
TO µP
100kΩ
100kΩ
2N3904
MR
PFO
+5V
0V
+5V
0V
VTRIP 0V
V-
VPFT = 2.5V (K, L, N); 1.70V (T AND MAX816)
5 - 2.5 = 2.5 - VTRIP , VTRIP < 0V
R1 R2
BEGIN PROGRAM
SET LOW
WDI
SET HIGH
WDI
SUBROUTINE
SET LOW
WDI
YES
NO
RETURN
Figure 14. Interfacing to µPs with Bidirectional Reset I/O
MAX814
MAX815
MAX816
BUFFERED RESET TO OTHER SYSTEM COMPONENTS
4.7kΩ
VCC
GND
RESET
VCC
GND
RESET
µP
—
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
______________________________________________________________________________________ 15
______________Ordering Information ___________________Chip Information
PART* TEMP. RANGE PIN-PACKAGE
MAX814_CPA 0°C to +70°C 8 Plastic DIP
MAX814_CSA 0°C to +70°C 8 SO
MAX814_EPA -40°C to +85°C 8 Plastic DIP
MAX814_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
*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°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
TRANSISTOR COUNT: 744
_____________Reset Trip Thresholds
MAX (V)
3.063.00T
4.604.50N
4.754.65L
4.854.75K
MAX814/MAX815
MIN (V)
RESET TRIP THRESHOLD
MAX816
SUFFIX
Adjustable—
Revision History
Pages changed at Rev 1: 1–4, 12-16.
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
DIM
A
A1
A2
A3
B
B1
C
D1
E
E1
e
eA
eB
L
MIN
–
0.015
0.125
0.055
0.016
0.045
0.008
0.005
0.300
0.240
0.100
0.300
–
0.115
MAX
0.200
–
0.175
0.080
0.022
0.065
0.012
0.080
0.325
0.310
–
–
0.400
0.150
MIN
–
0.38
3.18
1.40
0.41
1.14
0.20
0.13
7.62
6.10
2.54
7.62
–
2.92
MAX
5.08
–
4.45
2.03
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
D
D
D
D
D
D
PKG.
P
P
P
P
P
N
MIN
0.348
0.735
0.745
0.885
1.015
1.14
MAX
0.390
0.765
0.765
0.915
1.045
1.265
MIN
8.84
18.67
18.92
22.48
25.78
28.96
MAX
9.91
19.43
19.43
23.24
26.54
32.13
INCHES MILLIMETERS
PINS
8
14
16
18
20
24
C
AA2
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
0.053
0.004
0.014
0.007
0.150
0.228
0.016
MAX
0.069
0.010
0.019
0.010
0.157
0.244
0.050
MIN
1.35
0.10
0.35
0.19
3.80
5.80
0.40
MAX
1.75
0.25
0.49
0.25
4.00
6.20
1.27
INCHES MILLIMETERS
21-0041A
Narrow SO
SMALL-OUTLINE
PACKAGE
(0.150 in.)
DIM
D
D
D
MIN
0.189
0.337
0.386
MAX
0.197
0.344
0.394
MIN
4.80
8.55
9.80
MAX
5.00
8.75
10.00
INCHES MILLIMETERS
PINS
8
14
16
1.270.050
L
0°-8°
HE
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.
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go to www.maxim-ic.com/packages.)
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