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STM703 Datasheet(PDF) 9 Page - STMicroelectronics

Part No. STM703
Description  5V Supervisor with Battery Switchover
Download  37 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

STM703 Datasheet(HTML) 9 Page - STMicroelectronics

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Reset Output
819 Supervisor asserts a reset signal to the MCU
whenever VCC goes below the reset threshold
(VRST), a watchdog time-out occurs, or when the
Push-button Reset Input (MR) is taken low. RST is
guaranteed to be a logic low (logic high for
STM805) for 0V < VCC < VRST if VBAT is greater
than 1V. Without a back-up battery, RST is guar-
anteed valid down to VCC =1V.
During power-up, once VCC exceeds the reset
threshold an internal timer keeps RST low for the
reset time-out period, trec. After this interval RST
returns high.
If VCC drops below the reset threshold, RST goes
low. Each time RST is asserted, it stays low for at
least the reset time-out period (trec). Any time VCC
goes below the reset threshold the internal timer
clears. The reset timer starts when VCC returns
above the reset threshold.
Push-button Reset Input (STM703/704/819)
A logic low on MR asserts reset. Reset remains
asserted while MR is low, and for trec (see Figure
42., page 28) after it returns high. The MR input
has an internal 40k
Ω pull-up resistor, allowing it to
be left open if not used. This input can be driven
with TTL/CMOS-logic levels or with open-drain/
collector outputs. Connect a normally open mo-
mentary switch from MR to GND to create a man-
ual reset function; external debounce circuitry is
not required. If MR is driven from long cables or
the device is used in a noisy environment, connect
a 0.1µF capacitor from MR to GND to provide ad-
ditional noise immunity. MR may float, or be tied to
VCC when not used.
Watchdog Input (NOT available on STM703/
The watchdog timer can be used to detect an out-
of-control MCU. If the MCU does not toggle the
Watchdog Input (WDI) within tWD (1.6sec typ), the
reset is asserted. The internal watchdog timer is
cleared by either:
a reset pulse, or
by toggling WDI (high-to-low or low-to-high),
which can detect pulses as short as 50ns. If
WDI is tied high or low, a reset pulse is
triggered every 1.8sec (tWD + trec).
The timer remains cleared and does not count for
as long as reset is asserted. As soon as reset is re-
leased, the timer starts counting (see Figure
43., page 28).
Note: The watchdog function may be disabled by
floating WDI or tri-stating the driver connected to
WDI. When tri-stated or disconnected, the maxi-
mum allowable leakage current is 10uA and the
maximum allowable load capacitance is 200pF.
Note: Input frequency greater than 20ns (50MHz)
will be filtered.

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