SGM811 Datasheet, PDF (7/9 Pages) SG Micro Corp – Microprocessor Supervisory with Manual Reset Input

APPLICATION NOTES
MANUAL RESET
The SGM811/SGM812 is equipped with a manual reset input
This input is designed to operate in a noisy environment
where unwanted glitches could be induced. These glitches
could be produced by the bouncing action of a switch contact,
or where a manual reset switch may be located some distance
away from the circuit (the cabling of which may pick-up
noise).
The manual reset input is guaranteed to ignore logically valid
inputs that are faster than 100 ns and to accept inputs longer in
duration than 10 µs.
BENEFITS OF A VERY ACCURATE RESET THRESHOLD
Because the SGM811/SGM812 can operate effectively even
when there are large degradations of the supply voltages, the
possibility of a malfunction during a power failure is greatly
reduced. Another advantage of the SGM811/SGM812 is its
very accurate internal voltage reference circuit. Combined,
these benefits produce an exceptionally reliable
microprocessor supervisory circuit.
VCC
VCC
SGM811
RESET
GND
Figure 1. Ensuring a Valid RESET Output Down to VCC = 0V
ENSURING A VALID RESET OUTPUT DOWN TO VCC = 0 V
When VCC falls below1.0 V, the SGM811/SGM812’s RESET no
longer sinks current. Therefore, a high impedance CMOS logic
input connected to RESET may drift to undetermined logic
levels. To eliminate this problem, a 100 k Ω resistor should be
connected from RESET to ground.
RESET OUTPUT
On power-up and after VCC rises above the reset threshold, an
internal timer holds the reset output active for 240 ms (typical).
This is intended as a power-on reset signal for the processor. It
allows time for both the power supply and the microprocessor
to stabilize after power-up. If a power supply brownout or
interruption occurs, the reset output is similarly activated and
remains active for 240 ms (typical) after the supply recovers.
This allows time for the power supply and microprocessor to
stabilize.
The SGM811 provides an active low reset output ( RESET )
while the SGM812 provides an active high output ( RESET ).
During power-down of the SGM811, the RESET output
remains valid (low) with VCC as low as 1 V. This ensures that
the microprocessor is held in a stable shutdown condition as
the supply falls and also ensures that no spurious activity can
occur via the microprocessor as it powers up.
Glitch Immunity
The SGM811/SGM812 contains internal filtering circuitry
providing glitch immunity from fast transient glitches on the
power supply line.
VCC
VREF
VREF
VREF
VREF
RESET
t1
t1
t1= _RESET Time = 240ms Typical
VREF = _RESET Voltage Threshold
Figure 2. Power Fail RESET Timing
INTERFACING TO OTHER DEVICES
Output
The SGM811/SGM812 is designed to integrate with as many
devices as possible. One feature of the SGM811/SGM812 is the
reset output, which is directly proportional to VCC (this is
guaranteed only while VCC is greater than 1 V). This enables
the part to be used with both 3 V and 5 V, or any nominal
voltage within the minimum and maximum specifications for
VCC.
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SGM811/812