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SP810 Datasheet PDF : 8 Pages
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SP809 / SP810
3 Pin Microprocessor Supervisor Circuit
THEORY OF OPERATION
μP will be activated at a valid reset state.
These μP supervisory circuits assert reset to
prevent code execution errors during power-
up, power-down, or brownout conditions.
Reset is guaranteed to be a logic low for
VTH>VCC>0.9V. Once VCC exceeded the reset
threshold, an internal timer keeps RESET low
for the reset timeout period; after this interval,
RESET goes high.
If a brownout condition occurs (VCC drops
below the reset threshold), RESET goes low.
Any time VCC goes below the reset threshold,
the internal timer resets to zero, and RESET
goes low. The internal timer is activated after
VCC returns above the reset threshold, and
RESET remains low for the reset timeout
period.
BENEFIT OF HIGHLY ACCURATE RESET
THRESHOLD
SP809/810 with specified voltage as 5V±10%
or 3V±10% are ideal for systems using a
5V±5% or 3V±5% power supply. The reset is
guaranteed to assert after the power supply
falls below the minimum specified operating
voltage range of the system ICs. The pre-
trimmed thresholds are reducing the range
over which an undesirable reset may occur.
APPLICATION INFORMATION
NEGATIVE GOING VCC TRANSIENTS
In addition to issuing a reset to the µP during
power-up, power-down, and brownout
conditions, SP809 series are relatively
resistant to short-duration negative-going VCC
transient.
ENSURING A VALID RESET OUTPUT DOWN
TO VCC=0
When VCC falls below 0.9V, SP809 RESET
output no longer sinks current; it becomes an
open circuit. In this case, high-impedance
CMOS logic inputs connecting to RESET can
drift to undetermined voltages. Therefore,
SP809/810 with CMOS is perfect for most
applications of VCC down to 0.9V.
However in applications where RESET must be
valid down to 0V, adding a pull-down resistor
to RESET causes any leakage currents to flow
to ground, holding RESET low.
INTERFACING TO µP WITH BIDIRECTIONAL
RESET PINS
The RESET output on the SP809N is open
drain, this device interfaces easily with μPs
that have bidirectional reset pins. Connecting
the μP supervisor's RESET output directly to
the microcontroller's RESET pin with a single
pull-up resistor allows either device to assert
reset.
TEST CIRCUIT
Fig. 10: Test Circuit
© 2011 Exar Corporation
6/8
Rev. 2.0.0
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