SP706 SIPEX [Sipex Corporation], SP706 Datasheet - Page 12

• Manufacturer Part Number: SP706
• Description: Low Power Microprocessor Supervisory Circuits
• Manufacturer: SIPEX [Sipex Corporation]

sensitivity to high-frequency noise on the
line being monitored. RESET can be used to
monitor voltages other than the +5V V
line. Connect PFO to MR to initiate a RESET
pulse when PFI drops below 1.25V. Figure 17
shows the SP705/706/707/708 configured to
assert RESET when the +5V supply falls below
the RESET threshold, or when the +12V supply
falls below approximately 11V.
Monitoring a Negative Voltage Supply
The power-fail comparator can also monitor a
negative supply rail, shown in Figure 18. When
the negative rail is good (a negative voltage of
large magnitude), PFO is LOW. By adding the
resistors and transistor as shown, a HIGH PFO
triggers RESET. As long as PFO remains HIGH,
the SP705-708/813L/813M will keep RESET
asserted (where RESET = LOW and RESET =
HIGH). Note that this circuit's accuracy de-
pends on the PFI threshold tolerance, the V
line, and the resistors.
SP705DS/09
Figure 18. Monitoring a Negative Voltage Supply
PFO
100K
MR
2N3904
10K
to P
+5V
+5V
0V
0V
R
R
1
2
100K
10K
=
1.25 - V
RESET
5.0 - 1.25
PFO
MR
V
TRIP
TRIP
+5V
, V
V
GND
CC
0V
TRIP
< 0
PFI
SP705 Low Power Microprocessor Supervisory Circuits
V-
R
R
V-
V-
1
2
CC
CC
12
Interfacing to mPs with Bidirectional
RESET Pins
Motorola 68HC11 series, can contend with the
SP705/706/707/708 RESET output. If, for
example, the RESET output is driven HIGH and
the
logic levels may result. To correct this, connect
a 4.7K resistor between the RESET output and
the P reset I/O, as shown if Figure 19. Buffer
the RESET output to other system components.
Figure 19. Interfacing to Microprocessors with
Bidirectional RESET I/O for the SP705/706/707/708
Ps with bidirectional RESET pins, such as the
Buffered RESET connects to System Components
+5V
P wants to pull it LOW, indeterminate
GND
V
CC
RESET
4.7K
© Copyright 2000 Sipex Corporation
RESET
+5V
GND
V
P
CC