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APX803-26SR Datasheet(PDF) 7 Page - Diodes Incorporated

Part No. APX803-26SR
Description  er3-PIN MICROPROCESSOR RESET CIRCUIT
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Maker  DIODES [Diodes Incorporated]
Homepage  http://www.diodes.com
Logo DIODES - Diodes Incorporated

APX803-26SR Datasheet(HTML) 7 Page - Diodes Incorporated

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APX803/D
3-PIN MICROPROCESSOR RESET CIRCUIT
APX803
Document number: DS32132 Rev. 2 - 2
7 of 11
www.diodes.com
September 2010
© Diodes Incorporated
Timing Diagram
Vth
Vth
/RESET
RESET
TDelay
TDelay
TDelay
TDelay
Vcc
Vth
Functional Description
Microprocessors (µPs) and microcontrollers (µC) have a
reset input to ensure that it starts up in a known state.
The APX803/D drive the µP’s reset input to prevent
code-execution errors during power-up, power-down, or
brownout
conditions.
They
assert
a
reset
signal
whenever the VCC supply voltage declines below a
preset threshold and keep it asserted for a fixed period
of time after VCC has risen above the reset threshold. For
the APX803D this period is a minimum of 1ms while for
other APX803 variants it is at least 140ms. The
APX803/D have an open-drain output stage.
Ensuring a Valid Reset Output
Down to VCC = 0
RESET is guaranteed to be a logic low for VCC > 1V.
Once VCC exceeds 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 dips below the RESET 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 starts after VCC returns
above the reset threshold, and RESET remains low for
the reset timeout period.
When VCC falls below 1V, the APX803/D
RESET
output no longer sinks current — it becomes an open
circuit. Therefore, high-impedance CMOS logic inputs
connected to RESET can drift to undetermined voltages.
This presents no problem in most applications since
most µP and other circuitry is inoperative with VCC below
1V.
Interfacing to µP with Bidirectional Reset Pins
Since the RESET output on the APX803/D is open drain,
this device interfaces easily with
μP/µC that have
bidirectional reset pins, such as the Motorola 68HC11.
Connecting the
μP supervisor’s RESET output directly to
the microcontroller’s (
μC’s) RESET pin with a single pull-
up resistor allows either device to assert reset.
Supervising and monitoring Multiple Supplies
Generally,
the
pull-up
resistor
connected
to
the
APX803/D will connect to the supply voltage that is being
monitored at the IC’s VCC pin. However, some systems
may use the APX803/D open-drain output to level-shift
from the monitored supply to reset the µP powered by a
different supply voltage or monitor multiple supplies that
will be fed into 1 µC/µP reset input.


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