Low-Cost, µP Supervisory Circuits
MAX705–MAX708/MAX813L
WDI
6
WATCHDOG
TRANSITION
DETECTOR
V
CC
250µA
WATCHDOG
TIMER
TIMEBASE FOR
RESET AND
WATCHDOG
RESET
GENERATOR
8
WDO
V
CC
250µA
MR
1
RESET
GENERATOR
8
RESET
MR
1
2
7
7
V
CC
RESET
(RESET)
V
CC
2
RESET
4.65V*
4.65V*
PFI
4
MAX705
MAX706
MAX813L
PFI
5
PFO
4
MAX707
MAX708
5
PFO
1.25V
1.25V
* 4.40V FOR MAX7O6.
( ) ARE FOR MAX813L ONLY.
3
GND
* 4.40V FOR MAX7O6.
3
GND
Figure 1. MAX705/MAX706/MAX813L Block Diagram
Figure 2. MAX707/MAX708 Block Diagram
_______________Detailed Description
Reset Output
A microprocessor’s (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 MAX705-MAX708/MAX813L
assert reset during power-up and prevent code execu-
tion errors during power-down or brownout conditions.
On power-up, once V
CC
reaches 1V,
RESET
is a guaran-
teed logic low of 0.4V or less. As V
CC
rises,
RESET
stays
low. When V
CC
rises above the reset threshold, an inter-
nal timer releases
RESET
after about 200ms.
RESET
puls-
es low whenever V
CC
dips below the reset threshold, i.e.
brownout condition. If brownout occurs in the middle of
a previously initiated reset pulse, the pulse continues for
at least another 140ms. On power-down, once V
CC
falls
below the reset threshold,
RESET
stays low and is guar-
anteed to be 0.4V or less until V
CC
drops below 1V.
The MAX707/MAX708/MAX813L active-high RESET output
is simply the complement of the
RESET
output, and is
guaranteed to be valid with V
CC
down to 1.1V. Some µPs,
such as Intel’s 80C51, require an active-high reset pulse.
WDI input is three-stated, the watchdog timer will stay
cleared and will not count. As soon as reset is released
and WDI is driven high or low, the timer will start counting.
Pulses as short as 50ns can be detected.
Typically,
WDO
will be connected to the non-maskable
interrupt input (NMI) of a µP. When V
CC
drops below
the reset threshold,
WDO
will go low whether or not the
watchdog timer has timed out yet. Normally this would
trigger an NMI interrupt, but
RESET
goes low simultane-
ously, and thus overrides the NMI interrupt.
If WDI is left unconnected,
WDO
can be used as a low-
line output. Since floating WDI disables the internal
timer,
WDO
goes low only when V
CC
falls below the
reset threshold, thus functioning as a low-line output.
The MAX705/MAX706 have a watchdog timer and a
RESET
output. The MAX707/MAX708 have both active-
high and active-low reset outputs. The MAX813L has
both an active-high reset output and a watchdog timer.
Manual Reset
The manual-reset input (MR) allows reset to be triggered
by a pushbutton switch. The switch is effectively
debounced by the 140ms minimum reset pulse width.
MR
is TTL/CMOS logic compatible, so it can be driven by
an external logic line.
MR
can be used to force a watch-
dog timeout to generate a reset pulse in the MAX705/
MAX706/MAX813L. Simply connect
WDO
to
MR.
Watchdog Timer
The MAX705/MAX706/MAX813L watchdog circuit moni-
tors the µP’s activity. If the µP does not toggle the watch-
dog input (WDI) within 1.6sec and WDI is not three-stat-
ed,
WDO
goes low. As long as
RESET
is asserted or the
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