+5V, Low-Power µP Supervisory Circuits
with Adjustable Reset/Watchdog
MAX6301–MAX6304
_______________Detailed Description
Reset Function/Output
The reset output is typically connected to the reset
input of a microprocessor (µP). A µP’s reset input starts
or restarts the µP in a known state. The MAX6301–
MAX6304 µP supervisory circuits provide the reset
logic to prevent code-execution errors during power-
up, power-down, and brownout conditions (see
Typical
Operating Circuit).
For the MAX6301/MAX6303,
RESET
changes from high
to low whenever the monitored voltage (V
IN
) drops
below the reset threshold voltage (V
RST
).
RESET
remains low as long as V
IN
is below V
RST
. Once V
IN
exceeds V
RST
,
RESET
remains low for the reset timeout
period, then goes high. When a reset is asserted due to
a watchdog timeout condition,
RESET
stays low for the
reset timeout period. Anytime reset asserts, the watch-
dog timer clears. At the end of the reset timeout period,
RESET
goes high and the watchdog timer is restarted
from zero. If the watchdog timeout period is exceeded
again, then
RESET
goes low again. This cycle contin-
ues unless WDI receives a transition.
On power-up, once V
CC
reaches 1V,
RESET
is guaran-
teed to be a logic low. For information about applica-
tions where V
CC
is less than 1V, see the section
Ensuring a Valid
RESET/RESET
Output Down to V
CC
=
0V (MAX6303/MAX6304).
As V
CC
rises,
RESET
remains
low. When V
IN
rises above V
RST
, the reset timer starts
and
RESET
remains low. When the reset timeout period
ends,
RESET
goes high.
On power-down, once V
IN
goes below V
RST
,
RESET
goes low and is guaranteed to be low until V
CC
droops
below 1V. For information about applications where
V
CC
is less than 1V, see the section
Ensuring a Valid
RESET/RESET
Output Down to V
CC
= 0V (MAX6303/
MAX6304).
The MAX6302/MAX6304 active-high RESET output is
the inverse of the MAX6301/MAX6303 active-low
RESET
output, and is guaranteed valid for V
CC
> 1.31V.
R1
RESET IN
R2
V
CC
0.1µF
V
IN
MAX6301
MAX6302
MAX6303
MAX6304
V
RST
= 1.22
+
(
R1R2R2
)
Figure 1. Calculating the Reset Threshold Voltage (V
RST
)
R2 can have very high values to minimize current con-
sumption. Set R2 to some conveniently high value (1MΩ,
for example) and calculate R1 based on the desired
reset threshold voltage, using the following formula:
V
RST
R1
=
R2
×
−
1
Ω
V
TH
( )
Watchdog Timer
The watchdog circuit monitors the µP’s activity. If the µP
does not toggle the watchdog input (WDI) within t
WD
(user selected), reset asserts. The internal watchdog
timer is cleared by reset, by a transition at WDI (which
can detect pulses as short as 30ns), or by a transition
at WDS. The watchdog timer remains cleared while
reset is asserted; as soon as reset is released, the timer
starts counting (Figure 2).
The MAX6301–MAX6304 feature two modes of watch-
dog timer operation: normal mode and extended mode.
In normal mode (WDS = GND), the watchdog timeout
period is determined by the value of the capacitor con-
nected between SWT and ground (see the section
Selecting the Reset and Watchdog Timeout Capacitor).
In extended mode (WDS = V
CC
), the watchdog timeout
period is multiplied by 500. For example, in the extend-
ed mode, a 1µF capacitor gives a watchdog timeout
period of 22 minutes (see the graph Extended-Mode
Watchdog Timeout Period vs. C
SWT
in the
Typical
Operating Characteristics).
In extended mode, the watchdog function can be dis-
abled by leaving WDI unconnected or by three-stating
the driver connected to WDI. In this mode, the watch-
dog input is internally driven low during the watchdog
Reset Threshold
These supervisors monitor the voltage on RESET IN.
The MAX6301–MAX6304 have an adjustable reset
threshold voltage (V
RST
) set with an external resistor
voltage divider (Figure 1). Use the following formula to
calculate V
RST
(the point at which the monitored volt-
age triggers a reset):
V
TH
×
R1
+
R2
V
RST
=
V
R2
where V
RST
is the desired reset threshold voltage and
V
TH
is the reset input threshold (1.22V). Resistors R1 and
(
)
( )
6
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