SP708REP データシートの表示（PDF） - Exar Corporation

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SP708REP

+3.0V/+3.3V low power microprocessor supervisory circuits

Exar Corporation 

SP706R/S/T - SP708R/S/T

3.0V/3.3V Low Power Microprocessor Supervisory

Circuits

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 VCC 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

SP706/708 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

© 2015 Exar Corporation

Fig. 23: Interfacing to Microprocessors

with Bidirectional RESET I/O (SP706)

NEGATIVE-GOING VCC 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 VCC 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 VCC 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 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

13/17

Rev. 3.0.0

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