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STM1001 Datasheet(PDF) 4 Page - STMicroelectronics

Part No. STM1001
Description  Reset Circuit
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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STM1001 Datasheet(HTML) 4 Page - STMicroelectronics

 
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STM1001
4/13
OPERATION
Reset Output
The
STM1001
MICROPROCESSOR
RESET
CIRCUIT asserts a reset signal to the MCU when-
ever VCC goes below the reset threshold (VRST).
RST is guaranteed valid down to VCC =1V (0° to
70°C).
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. Any time VCC goes
below the reset threshold, the internal timer clears.
The reset timer starts when VCC returns above the
reset threshold. The active-low reset (RST) is an
open drain output.
Negative-Going VCC Transients
The STM1001 is relatively immune to negative-go-
ing VCC transients (glitches). Figure 11., page 7
shows typical transient duration versus reset com-
parator overdrive (for which the STM1001 will
NOT generate a reset pulse). The graph was gen-
erated using a negative pulse applied to VCC,
starting at 0.5V above the actual reset threshold
and ending below it by the magnitude indicated
(comparator overdrive). The graph indicates the
maximum pulse width a negative VCC transient
can have without causing a reset pulse. As the
magnitude of the transient increases (further be-
low the threshold), the maximum allowable pulse
width decreases. Any combination of duration and
overdrive which lies under the curve will NOT gen-
erate a reset signal. Typically, a VCC transient that
goes 100mV below the reset threshold and lasts
20µs or less will not cause a reset pulse. A 0.1µF
bypass capacitor mounted as close as possible to
the VCC pin provides additional transient immunity.
Valid /RST Output Down to VCC = 0V
When VCC falls below 1V, the RST output no long-
er sinks current, but becomes an open circuit. In
most systems this is not a problem, as most MCUs
do not operate below 1V. However, in applications
where RST output must be valid down to 0V, a
pull-down resistor may be added to hold the RST
output low. This resistor must be large enough to
not load the RST output, and still be small enough
to pull the output to ground. A 100K
Ω resistor is
recommended.


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