STM1815R STMICROELECTRONICS [STMicroelectronics], STM1815R Datasheet - Page 7

• Manufacturer Part Number: STM1815R
• Description: Low Power Reset Circuit
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

OPERATION
Reset Output
The STM181x asserts a reset signal to the Micro-
controller (MCU) whenever V
reset threshold (V
down to V
ler’s (MCU) reset input starts the MCU in a known
state. The STM1810 - STM1813/ STM1815 -
STM1818 Low Power Reset circuits assert reset to
prevent code-execution errors during power-up,
power-down, and brownout conditions
8., page
During power-up, once V
threshold an internal timer keeps RST low for the
reset time-out period, t
returns high.
If V
low. Each time RST is asserted, it stays low for at
least the reset time-out period. Any time V
below the reset threshold, the internal timer clears.
The reset timer starts when V
reset threshold. Reset t
externally initiated rising edge on the RST pin
(STM1813/STM1818), following a low signal of
1.5µs minimum duration.
Push-Button Detect Reset (STM1813/1818)
Many systems require push-button reset capability
(Figure 9., page
logic circuitry to initiate reset. On the STM1813/
STM1818, a logic low on RST held for greater than
1.5µs asserts a reset. RST deasserts following a
100ms minimum reset time-out delay (t
manual reset input shorter than 1.5µs may release
RST without the 100ms minimum reset time-out
delay. To facilitate use with mechanical switches,
the STM1813/STM1818 contain internal de-
bounce circuitry. A debounced waveform is shown
in
nal 5.5k
Interfacing to Bidirectional Microcontrollers
(MCU’s)
As the RST output on the STM1811/STM1816 is
open drain, these devices interface easily with
MCU’s that have bidirectional reset pins. Connect-
Figure 8. Reset Timing Diagram
Note: 1. RST for STM1812 and STM1817
Figure 10., page 8
CC
drops below the reset threshold, RST goes
7).
RST
RST
V CC
CC
pull-up resistor.
(1)
= 1.0V (0° to 105°C). A microcontrol-
V CC (min)
8), allowing the user or external
RST
The RST output has an inter-
), and is guaranteed valid
rec
rec
. After this interval, RST
CC
is also triggered by an
V RST
CC
CC
exceeds the reset
returns above the
goes below the
CC
(Figure
rec
t rec
t rec
goes
). A
STM1810/1811/1812/1813/1815/1816/1817/1818
ing the µP supervisor’s reset (RST) output directly
to the microcontroller’s reset (RST) pin allows ei-
ther device to assert reset
external pull-up resistor is required, as it is within
the STM1811/STM1816.
Negative Going V
The STM181x are relatively immune to negative-
going
20., page 13
sus reset comparator overdrive (for which the
STM181x will NOT generate a reset pulse). The
graph was generated using a negative pulse ap-
plied to V
set threshold and ending below it by the
magnitude indicated (comparator overdrive). The
graph indicates the maximum pulse width a nega-
tive V
set pulse. As the magnitude of the transient
increases (further below the threshold), the maxi-
mum allowable pulse width decreases. Any com-
bination of duration and overdrive which lies under
the curve will NOT generate a reset signal. Typi-
cally, a V
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 V
vides additional transient immunity.
Valid RST Output Down to V
When V
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 (see
must be large enough to not load the RST output,
and still be small enough to pull the output to
ground. A 100k
Note: The same situation applies for the active-
high RST of the STM1810/1812. A 100k
resistor to V
valid for V
CC
CC
V
transient can have without causing a re-
CC
CC,
CC
CC
falls below 1V, the RST output no long-
CC
transient that goes 100mV below the
shows typical transient duration ver-
< 1.0V.
starting at 0.5V above the actual re-
should be used if RST must remain
Figure 12., page
transients
resistor is recommended.
CC
Transients
(Figure 11., page
(glitches).
CC
= 0V
9). This resistor
AI09653
CC
pin pro-
pull-up
Figure
8). No
7/20