Buffered RESET connects to System Components
1nF Capacitor
VOUT TO GND
VCC
VCC
µP
Above Line
RESET
RESET
RESET
Generated
NO
4.7KΩ
RESET
Generated
GND
GND
Figure 24. Interfacing to Microprocessors with
Bidirectional RESET I/O
Figure 25. Maximum Transient Duration without
Causing a Reset Pulse vs. Reset Comparator Overdrive
Monitoring an Additional Power Supply
Figure 25 shows maximum transient duration
vs. reset-comparator overdrive, for which reset
pulses are not generated. The data was generated
using negative-going VCC pulses, starting at
3.3V and ending below the reset threshold by
the magnitude indicated (reset comparator
overdrive).Thegraphshowsthemaximumpulse
width a negative-going VCC transient may
typically have without causing a reset pulse to
be issued. As the amplitude of the transient
increases (i.e. goes farther below the reset
threshold), the maximum allowable pulse width
decreases. Typically, a VCC transient that goes
100mV below the reset threshold and lasts for
40µs or less will not cause a reset pulse to be
issued.A100nFbypasscapacitormountedclose
to the VCC pin provides additional transient
immunity.
These µP supervisors can monitor either positive
or negative supplies using a resistor voltage
divider to PFI. PFO can be used to generate an
interrupt to the µP, as seen in Figure 23.
Interfacing to µPs with Bidirectional
Reset Pins
Any µPs with bidirectional reset pins, such as
the Motorola 68HC11 series, can interface with
the SP690_ and the SP802_ RESET outputs.
For example, if the RESET output is driven
HIGH and the µP wants to pull it LOW,
indeterminate logic levels may result. To correct
this, connect a 4.7kΩ resistor between the
RESET output and the µP reset I/O, as in
Figure 24. Buffer the RESET output to other
system components.
Negative-Going VCC Transients
While issuing resets to the µP during power-up,
power-down, and brownout conditions, these
supervisors are relatively immune to short-
duration negative-going VCC transients
(glitches). It is usually undesirable to reset the
µP when VCC experiences only small glitches.
SP690T/S/R DS/10 SP690T/S/R, 802/T/S/R, 804T/S/R, 805T/S/R Low Power Microprocessor Supervisory
© Copyright 2000 Sipex Corporation
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