SP706REU-L Datasheet, PDF (12/16 Pages) Exar Corporation – 3.0V/3.3V Low Power Microprocessor Supervisory Circuits

SP706R/S/T - SP708R/S/T
3.0V/3.3V Low Power Microprocessor Supervisory
Circuits
resistors and transistor as shown, a HIGH PFO
triggers RESET. As long as PFO remains HIGH,
the SP706R/S/T-SP708R/S/T series will keep
RESET asserted (where RESET = LOW and
RESET = HIGH). Note that this circuit's
accuracy depends on the PFI threshold
tolerance, the V_CC line, and the resistors.
INTERFACING TO µPS WITH BIDIRECTIONAL
RESET PINS
µPs with bidirectional RESET pins, such as the
Motorola 68HC11 series, can contend with the
SP705/706/707/708 RESET output. If, for
example, 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 shown if
Figure 23. Buffer the RESET output to other
system components.
Fig. 21: Monitoring +3.3V/+3.0V and +12V Power
Supplies
Fig. 22: Monitoring a Negative Voltage Supply
© 2010 Exar Corporation
Fig. 23: Interfacing to Microprocessors
with Bidirectional RESET I/O (SP706)
NEGATIVE-GOING V_CC TRANSIENT
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 V_CC experiences only small glitches.
Figure 24 shows maximum transient duration
vs. reset-comparator overdrive, for which
reset pulses are not generated. The data was
generated using negative-going V_CC pulses,
starting at 3.3V and ending below the reset
threshold by the magnitude indicated (reset
comparator overdrive). The graph shows the
maximum pulse width a negative-going V_CC
transient may typically have without causing a
reset pulse to be issued. As the amplitude of
the transient increases (i.e. goes farther below
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Rev. 2.0.0