19-3899; Rev 1; 11/05
Microprocessor Monitor
General Description
The MAX1232 microprocessor (µP) supervisory circuit
provides µP housekeeping and power-supply supervi-
sion functions while consuming only 1/10th the power of
the DS1232. The MAX1232 enhances circuit reliability
in µP systems by monitoring the power supply, monitor-
ing the software execution, and providing a debounced
manual reset input. The MAX1232 is a plug-in upgrade
of the Dallas DS1232.
A reset pulse of at least 250ms duration is supplied on
power-up, power-down, and low-voltage brownout
conditions (5% or 10% supply tolerances can be
selected digitally). Also featured is a debounced man-
ual reset input that forces the reset outputs to their
active states for a minimum of 250ms. A digitally pro-
grammable watchdog timer monitors software execu-
tion and can be programmed for timeout settings of
150ms, 600ms, or 1.2s. The MAX1232 requires no
external components.
Features
o
Consumes 1/10th the Power of the DS1232
o
Precision Voltage Monitor—Adjustable +4.5V
or +4.75V
o
Power-OK/Reset Pulse Width—250ms Min
o
No External Components
o
Adjustable Watchdog Timer—150ms, 600ms,
or 1.2s
o
Debounced Manual Reset Input for External
Override
o
Available in 8-Pin PDIP/SO and 16-Pin Wide SO
Packages
MAX1232
Ordering Information
PART
MAX1232C/D
MAX1232CPA
MAX1232CSA
TEMP RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
PIN-PACKAGE
Dice*
8 PDIP
8 SO
Applications
Computers
Controllers
Intelligent Instruments
Automotive Systems
Critical µP Power Monitoring
Ordering Information continued on last page.
*Contact
factory for dice specifications.
Devices in PDIP and SO packages 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. Lead free
not available for CERDIP package.
Pin Configurations
TOP VIEW
PB RST 1
8
7
V
CC
ST
RST
RST
Typical Operating Circuit
TD 2
MAX1232
TOL
3
6
5
GND 4
V
CC
TOL
5%/10%
TOLERANCE
SELECT
REF
RESET
GENERATOR
RST
DIP/SO
N.C. 1
RST
16 N.C.
15 V
CC
14 N.C.
PB RST 2
N.C. 3
TD 4
N.C. 5
PB RST
MAX1232
DEBOUNCE
MAX1232
13 ST
12 N.C.
11 RST
10 N.C.
9
RST
TD
WATCHDOG
TIMEBASE
SELECT
GND
WATCHDOG
TIMER
ST
TOL 6
N.C. 7
GND 8
WIDE SO
________________________________________________________________
Maxim Integrated Products
1
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.
Microprocessor Monitor
MAX1232
ABSOLUTE MAXIMUM RATINGS
Voltage on Any Pin (with respect to GND)……………-1V to +7V
Operating Temperature Range
C Suffix ………………………………………………0°C to +70°C
E Suffix.……………………………………………-40°C to +85°C
M Suffix .…………………………………………-55°C to +125°C
Storage Temperature Range……………………-65°C to +160°C
Lead Temperature (soldering, 10s)………………………+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Recommended DC Operating Conditions
(T
A
= T
MIN
to T
MAX
)
PARAMETER
Supply Voltage
ST
and
PBRST
Input High Level
(Note 1)
ST
and
PBRST
Input Low Level
SYMBOL
V
CC
V
IH
V
IL
CONDITIONS
MIN
4.5
2.0
-0.3
TYP
5.0
MAX
5.5
V
CC
+
0.3
+0.8
UNITS
V
V
V
DC Electrical Characteristics
(V
CC
= +4.5V to +5.5V, T
A
= T
MIN
to T
MAX
)
PARAMETER
Input Leakage
ST,
TOL
Output Current RST
Output Current RST,
RST
Operating Current (Note 2)
V
CC
5% Trip Point (Note 3)
V
CC
10% Trip Point (Note 3)
SYMBOL
I
IL
I
OH
I
OL
I
CC
V
CCTP
V
CCTP
TOL = GND
TOL = V
CC
4.50
4.25
V
OH
= 2.4V
V
OL
= 0.4V
CONDITIONS
MIN
-1.0
-1.0
2.0
-12
10
50
4.62
4.37
200
4.74
4.49
TYP
MAX
+1.0
UNITS
µA
mA
mA
µA
V
V
Capacitance (Note 4)
(T
A
= +25°C)
PARAMETER
Input Capacitance
ST,
TOL
Output Capacitance RST,
RST
SYMBOL
C
IN
C
OUT
CONDITIONS
MIN
TYP
MAX
5
7
UNITS
pF
pF
2
_______________________________________________________________________________________
Microprocessor Monitor
AC Electrical Characteristics
(V
CC
= +5V ±10%, T
A
= T
MIN
to T
MAX
)
PARAMETER
PBRST
(Note 5)
PBRST
Delay
Reset Active Time
ST
Pulse Width
ST
Timeout Period
V
CC
Fall Time (Note 4)
V
CC
Rise Time (Note 4)
V
CC
Detect to RST High and
RST
Low
V
CC
Detect to RST Low and
RST
Open (Note 6)
SYMBOL
t
PB
t
PBD
t
RST
t
ST
t
TD
t
F
t
B
t
BPD
t
BPU
Figure 4
Figure 4, TD pin = 0V
TD pin = open
TD pin = V
CC
Figure 5
Figure 6
Figure 7, V
CC
falling
Figure 8, V
CC
rising
250
610
Figure 3
Figure 3
CONDITIONS
MIN
20
1
250
75
62.5
250
500
10
0
100
1000
150
600
1200
250
1000
2000
µs
µs
ns
ms
ms
4
610
20
1000
TYP
MAX
UNITS
ms
ms
ms
ns
MAX1232
Note 1:
Note 2:
Note 3:
Note 4:
Note 5:
Note 6:
PBRST
is internally pulled up to V
CC
with an internal impedance of typically 40kΩ.
Measured with outputs open.
All voltages referenced to GND.
Guaranteed by desing.
PBRST
must be held low for a minimum of 20ms to guarantee a reset.
t
R
= 5µs.
Pin Description
PIN
WIDE SO
1, 3, 5, 7, 10,
12, 14, 16
2
4
6
8
DIP/SO
—
1
2
3
4
NAME
N.C.
PBRST
TD
TOL
GND
No Connection
Pushbutton Reset Input. A debounced active-low input that ignores pulses less than 1ms
in duration and is guaranteed to recognize inputs of 20ms or greater.
Time Delay Set. The watchdog timebase select input (t
TD
= 150ms for TD = 0V, t
TD
=
600ms for TD = open, t
TD
= 1.2s for TD = V
CC
).
Tolerance Input. Connect to GND for 5% tolerance or to V
CC
for 10% tolerance.
Ground
Reset Output (Active High). Goes active:
(1) If VCC falls below the selected reset voltage threshold
(2) If PBRST is forced low
(3) If ST is not strobed within the minimum timeout period
(4) During power-up
Reset Output (Active Low, Open-Drain). See RST.
Strobe Input. Input for watchdog timer.
The +5V Power-Supply Input
FUNCTION
9
5
RST
11
13
15
6
7
8
RST
ST
V
CC
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3
Microprocessor Monitor
MAX1232
Detailed Description
Power Monitor
A voltage detector monitors V
CC
and holds the reset
outputs (RST and
RST)
in their active states whenever
V
CC
is below the selected 5% or 10% tolerance (4.62V
or 4.37V, typically). To select the 5% level, connect
TOL to ground. To select the 10% level, connect TOL to
V
CC
. The reset outputs will remain in their active states
until V
CC
has been continuously in-tolerance for a mini-
mum of 250ms (the reset active time) to allow the
power supply and µP to stabilize.
The RST output both sinks and sources current, while
the
RST
output, an open-drain MOSFET, sinks current
only and must be pulled high.
Watchdog Timer
The microprocessor drives the
ST
input with an
input/output (I/O) line. The microprocessor must toggle
the
ST
input within a set period (as determined by TD)
to verify proper software execution. If a hardware or
software failure keeps
ST
from toggling within the mini-
mum timeout period—ST is activated only by falling
edges (a high-to-low transition)—the MAX1232 reset
outputs are forced to their active states for 250ms
(Figure 2). This typically initiates the microprocessor’s
power-up routine. If the interruption continues, new
reset pulses are generated each timeout period until
ST
is strobed. The timeout period is determined by the TD
input connection. This timeout period is typically 150ms
with TD connected to GND, 600ms with TD floating, or
1200ms with TD connected to V
CC
.
The software routine that strobes
ST
is critical. The
code must be in a section of software that executes fre-
quently enough so the time between toggles is less
than the watchdog timeout period. One common tech-
nique controls the microprocessor I/O line from two
sections of the program. The software might set the I/O
line high while operating in the foreground mode, and
set it low while in the background or interrupt mode. If
both modes do not execute correctly, the watchdog
timer issues reset pulses.
Pushbutton Reset Input
The MAX1232’s debounced manual reset input
(PBRST) manually forces the reset outputs into their
active states. The reset outputs go active after
PBRST
has been held low for a time t
PBD
, the pushbutton reset
delay time. The reset outputs remain in their active
states for a minimum of 250ms after
PBRST
rises above
V
IH
(Figure 3).
A mechanical pushbutton or an active logic signal can
drive the
PBRST
input. The debounced input ignores
input pulses less than 1ms and is guaranteed to recog-
nize pulses of 20ms or greater. The
PBRST
input has
an internal pullup to V
CC
of about 100µA; therefore, an
external pullup resistor is not necessary.
+5V
+5V
10kΩ
V
CC
PB RST
MAX1232
TD
ST
I/O
MICROPROCESSOR
RST
RESET
+8V
7805
3-TERMINAL
REGULATOR
+5V
0.1µF
V
CC
MAX1232
ST
RST
RESET
MICROPROCESSOR
I/O
GND
TOL
TD
TOL
GND
Figure 1. Pushbutton Reset
Figure 2. Watchdog Timer
4
_______________________________________________________________________________________
Microprocessor Monitor
MAX1232
t
ST
t
PB
ST
PB RST
t
PBD
V
IH
V
IL
t
RST
t
TD
NOTE:
t
SD
IS THE MAXIMUM ELAPSED TIME BETWEEN ST HIGH-TO-LOW
TRANSITIONS (ST IS ACTIVATED BY FALLING EDGES ONLY) WHICH WILL
KEEP THE WATCHDOG TIMER FROM FORCING THE RESET OUTPUTS
ACTIVE FOR A TIME OF t
RST
. t
TD
IS A FUNCTION OF THE VOLTAGE AT
THE TD PIN, AS TABULATED BELOW.
t
TD
TYP
150ms
250ms
1200ms
RST
RST
CONDITION
TD pin = 0V
TD pin = open
TD pin = V
CC
MIN
62.5ms
250ms
500ms
MAX
250ms
1000ms
2000ms
Figure 3. Pushbutton Reset. The debounced PBRST input
ignores input pulses less than 1ms and is guaranteed to rec-
ognize pulses of 20ms or greater.
Figure 4. Watchdog Strobe Input
t
F
V
CC
t
R
+4.75V
+4.75V
+4.25V
+4.25V
V
CC
Figure 5. Power-Down Slew Rate
Figure 6. Power-Up Slew Rate
_______________________________________________________________________________________
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