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參數(shù)資料
| 型號(hào)： | STM708TM6F |
| 廠商： | 意法半導(dǎo)體 |
| 英文描述： | 3V Supervisor |
| 中文描述： | 3V的監(jiān) |
| 文件頁(yè)數(shù)： | 9/26頁(yè) |
| 文件大小： | 251K |
| 代理商： | STM708TM6F |
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STM706T/S/R; STM706P; STM708T/S/R
OPERATION
Reset Output
The STM70x Supervisor asserts a reset signal to
the MCU whenever V
CC
goes below the reset
threshold (V
RST
), a watchdog time-out occurs (if
WDO is connected to MR), or when the Push-but-
ton Reset Input (MR) is taken low. RST is guaran-
teed to be a logic low (logic high for STM706P and
STM708T/S/R) for V
CC
< V
RST
down to V
CC
=1V
for T
A
= 0°C to 85°C.
During power-up, once V
CC
exceeds the reset
threshold an internal timer keeps RST low for the
reset time-out period, t
rec
. After this interval RST
returns high.
If V
CC
drops below the reset threshold, RST goes
low. Each time RST is asserted, it stays low for at
least the reset time-out period (t
rec
). Any time V
CC
goes below the reset threshold the internal timer
clears. The reset timer starts when V
CC
returns
above the reset threshold.
Push-button Reset Input
A logic low on MR asserts reset. Reset remains
asserted while MR is low, and for t
rec
(see
Figure
29., page 19
) after it returns high. The MR input
has an internal 40k
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
V
CC
when not used.
Watchdog Input (STM706T/S/R and STM706P)
The watchdog timer can be used to detect an out-
of-control MCU. If the MCU does not toggle the
Watchdog Input (WDI) within t
WD
(1.6sec), the
Watchdog Output pin (WDO) is asserted. The in-
ternal 1.6sec timer is cleared by either:
1.
a reset pulse, or
2.
by toggling WDI (high-to-low or low-to-high),
which can detect pulses as short as 50ns.
See
Figure 30., page 19
for STM706T/S/R and
STM706P.
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.
Watchdog Output (STM706T/S/R and
STM706P)
When V
CC
drops below the reset threshold, WDO
will go low even if the watchdog timer has not yet
timed out. However, unlike the reset output, WDO
goes high as soon as V
CC
exceeds the reset
threshold. WDO may be used to generate a reset
pulse by connecting it to the MR input.
Power-fail Input/Output
The Power-fail Input (PFI) is compared to an inter-
nal reference voltage (independent from the V
RST
comparator). If PFI is less than the power-fail
threshold (V
PFI
), the Power-Fail Output (PFO) will
go low. This function is intended for use as an un-
dervoltage detector to signal a failing power sup-
ply. Typically PFI is connected through an external
voltage divider (see
Figure 10., page 8
) to either
the unregulated DC input (if it is available) or the
regulated output of the V
CC
regulator. The voltage
divider can be set up such that the voltage at PFI
falls below V
PFI
several milliseconds before the
regulated V
CC
input to the STM70x or the micro-
processor drops below the minimum operating
voltage.
If the comparator is unused, PFI should be con-
nected to V
SS
and PFO left unconnected. PFO
may be connected to MR on the STM70x so that a
low voltage on PFI will generate a reset output.
Ensuring a Valid Reset Output Down to
V
CC
= 0V
When V
CC
falls below 1V, the state of the RST out-
put can no longer be guaranteed, and becomes
essentially an open circuit. If a high value pull-
down resistor is added to the RST pin, the output
will be held low during this condition. A resistor val-
ue of approximately 100k
will be large enough to
not load the output under operating conditions, but
still sufficient to pull RST to ground during this low
voltage condition (see Figure
11
).
Figure 11. Reset Output Valid to Ground
Circuit
AI08844
STM70x
RST
R1
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