MAXIM MAX6372

19-1676; Rev 1; 7/00
Pin-Selectable Watchdog Timers
• Watchdog output/timing options: open drain (100ms)
or push-pull (1ms).
The MAX6369–MAX6374 operate over a +2.5V to +5.5V
supply range and are available in miniature 8-pin
SOT23 packages.
________________________Applications
Embedded Control Systems
Industrial Controllers
Critical µP and Microcontroller (µC) Monitoring
Features
♦ Precision Watchdog Timer for Critical
µP Applications
♦ Pin-Selectable Watchdog Timeout Periods
♦ Pin-Selectable Watchdog Startup Delay Periods
♦ Ability to Change Watchdog Timing Characteristics
Without Power Cycling
♦ Open-Drain or Push-Pull Pulsed Active-Low
Watchdog Output
♦ Watchdog Timer Disable Feature
♦ +2.5V to +5.5V Operating Voltage
♦ 8µA Low Supply Current
♦ No External Components Required
♦ Miniature 8-Pin SOT23 Package
Ordering Information
PART
TEMP.
RANGE
PINPACKAGE
TOP
MARKS
MAX6369KA-T
-40°C to +85°C
8 SOT23-8
AADC
MAX6370KA-T
-40°C to +85°C
8 SOT23-8
AADD
MAX6371KA-T
-40°C to +85°C
8 SOT23-8
AADE
MAX6372KA-T
-40°C to +85°C
8 SOT23-8
AADF
MAX6373KA-T
-40°C to +85°C
8 SOT23-8
AADG
MAX6374KA-T
-40°C to +85°C
8 SOT23-8
AADH
Note: All devices are available in tape-and-reel only. Required
order increment is 2,500 pieces.
Automotive
Telecommunications
Pin Configuration appears at end of data sheet.
Networking
Selector Guide
PART
MAX6369
OUTPUT
WDO PULSE
WIDTH (ms)
Open Drain
100
MAX6370
Push-Pull
MAX6371
Open Drain
MAX6372
Push-Pull
MAX6373
Open Drain
MAX6374
Push-Pull
1
MINIMUM STARTUP DELAY
Selectable: 1ms to 60s
MINIMUM WATCHDOG TIMEOUT
Selectable: 1ms to 60s
Selectable: 1ms to 60s
Selectable: 1ms to 60s
100
60s
Selectable: 1ms to 60s
1
60s
Selectable: 1ms to 60s
100
Selectable: 200µs to 60s or first edge
Selectable: 30µs to 10s
1
Selectable: 200µs to 60s or first edge
Selectable: 30µs to 10s
________________________________________________________________ Maxim Integrated Products
1
For free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
MAX6369–MAX6374
General Description
The MAX6369–MAX6374 are pin-selectable watchdog
timers that supervise microprocessor (µP) activity and
signal when a system is operating improperly. During
normal operation, the microprocessor should repeatedly toggle the watchdog input (WDI) before the selected
watchdog timeout period elapses to demonstrate that
the system is processing code properly. If the µP does
not provide a valid watchdog input transition before the
timeout period expires, the supervisor asserts a watchdog (WDO) output to signal that the system is not executing the desired instructions within the expected time
frame. The watchdog output pulse can be used to reset
the µP or interrupt the system to warn of processing
errors.
The MAX6369–MAX6374 are flexible watchdog timer
supervisors that can increase system reliability through
notification of code execution errors. The family offers
several pin-selectable watchdog timing options to
match a wide range of system timing applications:
• Watchdog startup delay: provides an initial delay
before the watchdog timer is started.
• Watchdog timeout period: normal operating watchdog timeout period after the initial startup delay.
MAX6369–MAX6374
Pin-Selectable Watchdog Timers
ABSOLUTE MAXIMUM RATINGS
Terminal Voltage (with respect to GND)
VCC .....................................................................-0.3V to +6V
WDI.....................................................................-0.3V to +6V
WDO (Open Drain: MAX6369/71/73) .................-0.3V to +6V
WDO (Push-Pull: MAX6370/72/74 .......-0.3V to (VCC + 0.3V)
SET0, SET1, SET2 ................................-0.3V to (VCC + 0.3V)
Maximum Current, Any Pin (input/output)...........................20mA
Continuous Power Dissipation (TA = +70°C)
SOT23-8 (derate 8.75mW/°C above +70°C)...............700mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Junction Temperature ......................................................+150°C
Lead Temperature (soldering, 10s) .................................+300°C
VCC Rise or Fall Rate......................................................0.05V/µs
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VCC = +2.5V to +5.5V, SET_ = VCC or GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C and
VCC = +3V.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
Operating Voltage Range
VCC
Supply Current
ICC
No load
Input High Voltage
VIH
WDI, SET0, SET1, SET2
Input Low Voltage
VIL
Logic Input Current (Note 2)
WDO Output Low
Voltage
WDO Leakage Current
WDO Output High
Voltage
MIN
WDI, SET0, SET1,
SET2
8
ILKG
UNITS
5.5
V
20
µA
V
VCC ≥ 3.3V
0.8
VCC ≥ 2.5V
0.6
V
±10
nA
ISINK = 1.2mA, V CC > 2.7V, watchdog
output asserted
0.3
V
ISINK = 6mA, V CC > 4.5V, watchdog
output asserted
0.4
V
1
µA
0
V NDO = 0 to +5.5V, output deasserted,
MAX6369/MAX6371/MAX6373
ISOURCE = 500µA, VCC > 2.7V, watchdog
output deasserted
VOH
MAX
0.8 × VCC
WDI or SET_ = 0 or V CC
VOL
TYP
2.5
0.8 × VCC
V
ISOURCE = 800µA, VCC > 4.5V, watchdog
output deasserted
VCC - 1.5
MAX6369/MAX6370
SET2 = 0, SET1 = 0, SET0 = 0
1
3
SET2 = 0, SET1 = 0, SET0 = V CC
10
30
SET2 = 0, SET1 = V CC , SET0 = 0
30
90
SET2 = 0, SET1 = V CC , SET0 = V CC
Startup Delay Period
tDELAY
SET2 = V CC , SET1 = 0, SET0 = 0
SET2 = V CC , SET1 = 0, SET0 = V CC
2
ms
Watchdog Disabled
100
300
1
3
SET2 = V CC , SET1 = V CC , SET0 = 0
10
30
SET2 = V CC , SET1 = V CC , SET0 = V CC
60
180
_______________________________________________________________________________________
ms
s
Pin-Selectable Watchdog Timers
(VCC = +2.5V to +5.5V, SET_ = VCC or GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C and
VCC = +3V.) (Note 1)
PARAMETER
Watchdog Timeout
Period
SYMBOL
tWD
CONDITIONS
MIN
TYP
MAX
SET2 = 0, SET1 = 0, SET0 = 0
1
3
SET2 = 0, SET1 = 0, SET0 = VCC
10
30
SET2 = 0, SET1 = VCC, SET0 = 0
30
90
SET2 = 0, SET1 = VCC, SET0 = VCC
SET2 = VCC, SET1 = 0, SET0 = 0
UNITS
ms
Watchdog Disabled
100
300
SET2 = VCC, SET1 = 0, SET0 = VCC
1
3
SET2 = VCC, SET1 = VCC, SET0 = 0
10
30
SET2 = VCC, SET1 = VCC, SET0 = VCC
60
180
ms
s
MAX6371/MAX6372
Startup Delay Period
Watchdog Time-Out
Period
tDELAY
tWD
SET2 = 0, SET1 = VCC, SET0 = VCC
Watchdog Disabled
All other SET_ conditions
60
180
SET2 = 0, SET1 = 0, SET0 = 0
1
3
SET2 = 0, SET1 = 0, SET0 = VCC
3
9
SET2 = 0, SET1 = VCC, SET0 = 0
10
30
SET2 = 0, SET1 = VCC, SET0 = VCC
s
ms
Watchdog Disabled
SET2 = VCC, SET1 = 0, SET0 = 0
100
300
SET2 = VCC, SET1 = 0, SET0 = VCC
300
900
SET2 = VCC, SET1 = VCC, SET0 = 0
3
9
SET2 = VCC, SET1 = VCC, SET0 = VCC
60
180
SET2 = 0, SET1 = 0, SET0 = 0
3
9
ms
s
MAX6373/MAX6374
Startup Delay Period
Watchdog Timeout
Period
tDELAY
tWD
SET2 = 0, SET1 = 0, SET0 = VCC
3
9
SET2 = 0, SET1 = VCC, SET0 = 0
60
180
SET2 = 0, SET1 = VCC, SET0 = VCC
SET2 = VCC, SET1 = 0, SET0 = 0
ms
s
Watchdog Disabled
200
600
SET2 = VCC, SET1 = 0, SET0 = VCC
First Edge (Note 3)
SET2 = VCC, SET1 = VCC, SET0 = 0
First Edge (Note 3)
µs
SET2 = VCC, SET1 = VCC, SET0 = VCC
60
180
s
SET2 = 0, SET1 = 0, SET0 = 0
3
9
ms
SET2 = 0, SET1 = 0, SET0 = VCC
3
9
SET2 = 0, SET1 = VCC, SET0 = 0
1
3
SET2 = 0, SET1 = VCC, SET0 = VCC
s
Watchdog Disabled
SET2 = VCC, SET1 = 0, SET0 = 0
30
90
SET2 = VCC, SET1 = 0, SET0 = VCC
1
3
SET2 = VCC, SET1 = VCC, SET0 = 0
10
30
SET2 = VCC, SET1 = VCC, SET0 = VCC
10
30
µs
s
_______________________________________________________________________________________
3
MAX6369–MAX6374
ELECTRICAL CHARACTERISTICS (continued)
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.5V to +5.5V, SET_ = VCC or GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C and
VCC = +3V.) (Note 1)
PARAMETER
SYMBOL
Watchdog Input Pulse Width
(Note 2)
tWDI
Watchdog Output Pulse Width
tWDO
Internal Setup Time
(Note 4)
tSETUP
CONDITIONS
MIN
TYP
MAX
UNITS
After WDO deasserted
100
MAX6369/MAX6371/MAX6373
100
300
ms
MAX6370/MAX6372/MAX6374
1
3
ms
100
300
ms
After WDO deasserted
ns
Note 1: Production tested at TA = +25°C. Guaranteed by design over temperature limits.
Note 2: Guaranteed by design.
Note 3: In this setting the watchdog timer is inactive and startup delay ends when WDI sees its first level transition. See Selecting
Device Timing for more information.
Note 4: After power-up, or a setting change, there is an internal setup time during which WDI is ignored.
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
WATCHDOG TIMEOUT PERIOD
vs. TEMPERATURE
SUPPLY CURRENT
vs. TEMPERATURE
VCC = +5.5V
10
8
VCC = +2.5V
6
1.002
1.001
1.000
0.999
0.998
0.997
4
-40
-15
10
35
TEMPERATURE (°C)
4
MAX6369/74-02
12
1.003
NORMALIZED WATCHDOG TIMEOUT PERIOD
MAX6369/74-01
14
SUPPLY CURRENT (µA)
MAX6369–MAX6374
Pin-Selectable Watchdog Timers
60
85
-40
-15
10
35
TEMPERATURE (°C)
_______________________________________________________________________________________
60
85
Pin-Selectable Watchdog Timers
PIN
NAME
FUNCTION
1
WDI
Watchdog Input. If WDI remains either high or low for the duration of the watchdog timeout period (tWD), WDO
triggers a pulse. The internal watchdog timer clears whenever a WDO is asserted or whenever WDI sees a
rising or falling edge.
2
GND
Ground
3
N.C.
Not Connected. Do not make any connection to this pin.
4
SET0
Set Zero. Logic input for selecting startup delay and watchdog timeout periods. See Table 1 for timing details.
5
SET1
Set One. Logic input for selecting startup delay and watchdog timeout periods. See Table 1 for timing details.
6
SET2
Set Two. Logic input for selecting startup delay and watchdog timeout periods. See Table 1 for timing details.
7
WDO
Watchdog Output. Pulses low for the watchdog output pulse width, tWDO, when the internal watchdog times
out. The MAX6369/MAX6371/MAX6373 have open-drain outputs and require a pull-up resistor. The
MAX6370/MAX6372/MAX6374 outputs are push-pull.
8
VCC
Supply Voltage (+2.5V to +5.5V)
Table 1. Minimum Timeout Settings
LOGIC INPUTS
MAX6369/MAX6370
MAX6371/MAX6372
MAX6373/MAX6374
tDELAY, tWD
tDELAY = 60s, tWD
tDELAY
tWD
SET2
SET1
SET0
0
0
0
1ms
1ms
3ms
3ms
0
0
1
10ms
3ms
3s
3s
0
1
0
30ms
10ms
60s
1s
0
1
1
Disabled
Disabled
Disabled
Disabled
1
0
0
100ms
100ms
200µs
30µs
1
0
1
1s
300ms
First Edge
1s
1
1
0
10s
3s
First Edge
10s
1
1
1
60s
60s
60s
10s
Detailed Description
The MAX6369–MAX6374 are flexible watchdog circuits
for monitoring µP activity. During normal operation, the
internal timer is cleared each time the µP toggles the
WDI with a valid logic transition (low to high or high to
low) within the selected timeout period (tWD). The WDO
remains high as long as the input is strobed within the
selected timeout period. If the input is not strobed
before the timeout period expires, the watchdog output
is asserted low for the watchdog output pulse width
(tWDO). The device type and the state of the three logic
control pins (SET0, SET1, and SET2) determine watchdog timing characteristics. The three basic timing variations for the watchdog startup delay and the normal
watchdog timeout period are summarized below (see
Table 1 for the timeout characteristics for all devices in
the family):
• Watchdog Startup Delay:
Provides an initial delay before the watchdog timer is
started.
Allows time for the µP system to power up and initialize before assuming responsibility for normal watchdog timer updates.
Includes several fixed or pin-selectable startup delay
options from 200µs to 60s, and an option to wait for
the first watchdog input transition before starting the
watchdog timer.
_______________________________________________________________________________________
5
MAX6369–MAX6374
Pin Description
MAX6369–MAX6374
Pin-Selectable Watchdog Timers
WDI
VCC
MAX6369–MAX6374
TRANSITION
DETECTOR
SET 2
CLEAR
CONTROL
LOGIC
SET 1
EN
ENABLE
SET 0
SET WATCHDOG TIMEOUT
OUT
WDO
OUTPUT
SET STARTUP DELAY
WATCHDOG
TIMER CIRCUITRY
GND
Figure 1. Functional Diagram
VCC
tSETUP
tDELAY
tWD
tWD
tWD
WDI
A
A
B
C
E F
D
G
E
tWDO
WDO
A TRANSITIONS ON WDI IGNORED DURING SETUP DELAY.
B TRANSITION(S) ON WDI IGNORED DURING STARTUP DELAY PERIOD.
C WATCHDOG TIMER STARTS AFTER STARTUP DELAY AND WDO IS DEASSERTED.
D TRANSITION OCCURS BEFORE WATCHDOG TIMEOUT PERIOD. WATCHDOG
TIMER CLEARS AND STARTS TIMER AGAIN.
E WATCHDOG TIMES OUT, WDO ASSERTS.
F TRANSITIONS ON WDI IGNORED WHEN WDO ASSERTED.
G WATCHDOG TIMER STARTS AFTER WDO DEASSERTS.
Figure 2. Watchdog Timing
• Watchdog Timeout Period:
Normal operating watchdog timeout period after the
initial startup delay.
A watchdog output pulse is asserted if a valid watchdog input transition is not received before the timeout
period elapses.
Eight pin-selectable timeout period options for each
device, from 30µs to 60s.
Pin-selectable watchdog timer disable feature.
6
• Watchdog Output/Timing Options:
Open drain, active low with 100ms minimum watchdog output pulse (MAX6369/MAX6371/MAX6373).
Push-pull, active low with 1ms minimum watchdog
output pulse (MAX6370/MAX6372/MAX6374).
Each device has a watchdog startup delay that is initiated when the supervisor is first powered or after the
user modifies any of the logic control set inputs. The
watchdog timer will not begin to count down until the
_______________________________________________________________________________________
Pin-Selectable Watchdog Timers
delays from 3ms to 10s minimum. Two of the selectable
modes do not initiate the watchdog timer until the
device receives its first valid watchdog input transition
(there is no fixed period by which the first input must be
received). These two extended startup delay modes
are useful for applications requiring more than 60s for
system initialization.
All the MAX6369–MAX6374 devices may be disabled
with the proper logic control pin setting (Table 1).
Applications Information
Input Signal Considerations
Watchdog timing is measured from the last WDI rising
or falling edge associated with a pulse of at least 100ns
in width. WDI transitions are ignored when WDO is
asserted, and during the startup delay period (Figure
2). Watchdog input transitions are also ignored for a
setup period, tSETUP, of up to 300µs after power-up or
a setting change (Figure 3).
Selecting Device Timing
SET2, SET1, and SET0 program the startup delay and
watchdog timeout periods (Table 1). Timeout settings
can be hard wired, or they can be controlled with logic
gates and modified during operation. To ensure smooth
transitions, the system should strobe WDI immediately
before the timing settings are changed. This minimizes
the risk of initializing a setting change too late in the
timer countdown period and generating undesired
watchdog outputs. After changing the timing settings,
two outcomes are possible based on WDO. If the
change is made while WDO is asserted, the previous
setting is allowed to finish, the characteristics of the
new setting are assumed, and the new startup phase is
entered after a 300µs setup time (tSETUP) elapses. If
the change is made while WDO is not asserted, the
new setting is initiated immediately, and the new startup phase is entered after the 300µs setup time elapses.
The startup delay and the watchdog timeout period are
determined by the states of the SET0, SET1, and SET2
pins, and by the particular device within the family. For
the MAX6369 and MAX6370, the startup delay is equal
to the watchdog timeout period. The startup and
watchdog timeout periods are pin selectable from 1ms
to 60s (minimum).
For the MAX6371 and MAX6372, the startup delay is
fixed at 60s and the watchdog timeout period is pin
selectable from 1ms to 60s (minimum).
The MAX6373/MAX6374 provide two timing variations
for the startup delay and normal watchdog timeout.
Five of the pin-selectable modes provide startup delays
from 200µs to 60s minimum, and watchdog timeout
tWD
WDI
*
*
WDO
*
*
*
tWD
tWD
SET_
tSETUP
tDELAY
tWD
tSETUP
tDELAY
tWD
*IGNORED EDGE
Figure 3. Setting Change Timing
_______________________________________________________________________________________
7
MAX6369–MAX6374
completion of the startup delay period, and no watchdog output pulses will be asserted during the startup
delay. When the startup delay expires, the watchdog
begins counting its normal watchdog timeout period
and waiting for WDI transitions. The startup delay
allows time for the µP system to power up and fully initialize before assuming responsibility for the normal
watchdog timer updates. Startup delay periods vary
between the different devices and may be altered by
the logic control set pins. To ensure that the system
generates no undesired watchdog outputs, the routine
watchdog input transitions should begin before the
selected minimum startup delay period has expired.
The normal watchdog timeout period countdown is initiated when the startup delay is complete. If a valid logic
transition is not recognized at WDI before the watchdog
timeout period has expired, the supervisor asserts a
watchdog output. Watchdog timeout periods vary
between the different devices and may be altered by
the logic control set pins. To ensure that the system
generates no undesired watchdog outputs, the watchdog input transitions should occur before the selected
minimum watchdog timeout period has expired.
MAX6369–MAX6374
Pin-Selectable Watchdog Timers
to register a logic high while supplying all input current
and leakage paths connected to the WDO line. A 10kΩ
pull-up is sufficient in most applications. The MAX6370/
MAX6372/MAX6374 have push-pull outputs that provide an active-low watchdog output pulse of 1ms.
When WDO deasserts, timing begins again at the
beginning of the watchdog timeout period (Figure 2).
START
SET WDI
HIGH
Usage in Noisy Environments
If using the watchdog timer in an electrically noisy environment, a bypass capacitor of 0.1µF should be connected between VCC and GND as close to the device
as possible, and no further away than 0.2 inches.
PROGRAM
CODE
SUBROUTINE OR
PROGRAM LOOP
POSSIBLE
INFINITE LOOP PATH
SET WDI LOW
RETURN
Figure 4. Watchdog Flow Diagram
Selecting 011 (SET2 = 0, SET1 = 1, SET0 = 1) will disable the watchdog timer function on all devices in the
family. Operation may be reenabled without powering
down by changing the set inputs to the new desired
setting. The device will assume the new selected timing
characteristics and enter the startup phase after the
300µs setup time elapses (Figure 3).
The MAX6373/MAX6374 offer a first-edge feature. In
first-edge mode (settings 101 or 110, Table 1), the
internal timer does not control the startup delay period.
Instead, startup terminates when WDI sees a transition.
If changing to first-edge mode while the device is operating, disable mode must be entered first. It is then safe
to select first-edge mode. Entering disable mode first
ensures the output will be unasserted when selecting
first-edge mode and removes the danger of WDI being
masked out.
Output
The MAX6369/MAX6371/MAX6373 have an active-low,
open-drain output that provides a watchdog output
pulse of 100ms. This output structure will sink current
when WDO is asserted. Connect a pull-up resistor from
WDO to any supply voltage up to +5.5V.
Select a resistor value large enough to register a logic
low (see Electrical Characteristics), and small enough
________________Watchdog Software
Considerations
To help the watchdog timer monitor software execution
more closely, set and reset 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, in which the watchdog timer would
continue to be reset inside the loop, keeping the watchdog from timing out. Figure 4 shows an example of a
flow diagram where the I/O driving the watchdog input is
set high at the beginning of the program, set low at the
end of every subroutine or loop, then set high again
when the program returns to the beginning. If the program should hang in any subroutine, the problem would
be quickly corrected, since the I/O is continually set low
and the watchdog timer is allowed to time out, causing
WDO to pulse.
Pin Configuration
TOP VIEW
WDI
1
GND
2
N.C.
3
SET0 4
MAX6369
MAX6370
MAX6371
MAX6372
MAX6373
MAX6374
8
VCC
7
WDO
6
SET2
5
SET1
SOT23-8
Chip Information
TRANSISTOR COUNT: 1500
PROCESS: BiCMOS
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.
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