MAX824 MAXIM [Maxim Integrated Products], MAX824 Datasheet - Page 7

• Manufacturer Part Number: MAX824
• Description: 5-Pin Microprocessor Supervisory Circuits
• Manufacturer: MAXIM [Maxim Integrated Products]

The RESET output maximum pull-up current is 800µA
for L/M versions (400µA for T/S/R versions). This allows
µPs with bidirectional resets, such as the 68HC11, to
force RESET low when the MAX823/MAX824/MAX825
are pulling RESET high (Figure 4).
These supervisors are relatively immune to short-
duration, negative-going V
usually do not require the entire system to shut down.
Resets are issued to the µP during power-up, power-
down, and brownout conditions.
The Typical Operating Characteristics show a graph of
the MAX823L’s Maximum V
Reset Threshold Overdrive, for which reset pulses are
not generated. The graph was produced using nega-
tive-going V
the reset threshold by the magnitude indicated (reset
threshold overdrive). The graph shows the maximum
pulse width that a negative-going V
typically have without triggering a reset pulse. As the
amplitude of the transient increases (i.e., goes farther
below the reset threshold), the maximum allowable
pulse width decreases. Typically, a V
goes 100mV below the reset threshold and lasts for
15µs or less will not trigger a reset pulse.
Figure 4. Interfacing to µPs with Bidirectional Resets
GENERATOR
RESET
MAX823
MAX824
MAX825
V
CC
GND
CC
5-Pin Microprocessor Supervisory Circuits
Negative-Going V
pulses, starting at 5V and ending below
_______________________________________________________________________________________
V
CC
Bidirectional Reset Pins
Interfacing to µPs with
CC
I
SOURCE
CC
transients (glitches), which
MAX = 800 A L, M
Transient Duration vs.
400 A T, S, R
CC
CC
CC
Transients
transient can
transient that
RESET
GND
V
CC
P
An optional 0.1µF bypass capacitor mounted close to
V
One way to help the watchdog timer monitor software
execution more closely is to 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 5 shows an example of a flow diagram where the
I/O driving the watchdog input is set high at the begin-
ning of the program, set low at the beginning of every
subroutine or loop, then set high again when the pro-
gram returns to the beginning. If the program should
hang in any subroutine, the problem would quickly be
corrected, since the I/O is continually set low and the
watchdog timer is allowed to time out, causing a reset
or interrupt to be issued. As described in the Watchdog
Input Current section, this scheme results in higher time
average WDI input current than does leaving WDI low
for the majority of the timeout period and periodically
pulsing it low-high-low.
Figure 5. Watchdog Flow Diagram
CC
provides additional transient immunity.
Watchdog Software Considerations
SUBROUTINE OR
PROGRAM LOOP
SET WDI LOW
PROGRAM
SET WDI
RETURN
START
CODE
HIGH
(MAX823/MAX824)
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