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

16

Figure 24. Interfacing to Microprocessors with

Bidirectional RESET I/O

GND

GND

RESET

RESET

4.7K

Ω

µP

Buffered RESET connects to System Components

Monitoring an Additional Power Supply

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 4.7k

Ω 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 V

While issuing resets to the

µP during power-up,

power-down, and brownout conditions, these

supervisors are relatively immune to short-

duration negative-going V

CC

transients

(glitches). It is usually undesirable to reset the

µP when V

Figure 25 shows maximum transient duration

vs. reset-comparator overdrive, for which reset

pulses are not generated. The data was generated

using negative-going V

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

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 V

100mV below the reset threshold and lasts for

40

µs or less will not cause a reset pulse to be

issued. A 100nF bypass capacitor mounted close

to the V

immunity.

Figure 25. Maximum Transient Duration without

Causing a Reset Pulse vs. Reset Comparator Overdrive

1nF Capacitor

V

Above Line

RESET

Generated

NO

RESET

Generated