SS8037/8(G)
V
CC
V
CC
R
PULL-UP
V
CC
8038
RESET
µP
RESET
MOTOROLA
INPUT
68HCXX
Interfacing to µPs with Bidirectional Reset Pins
Since the
RESET
output on the SS8038L is open drain,
this device interfaces easily with µPs that have bidirec-
tional reset pins, such as the Motorola 68HC11. Con-
necting the µP supervisor’s
RESET
output directly to
the microcontroller’s (µC’s)
RESET
pin with a single
pull-up resistor allows either device to assert reset (Fig-
ure 3).
8038L Open-Drain
RESET
Output Allows Use with
Multiple Supplies
Generally, the pull-up connected to the SS8038L will
connect to the supply voltage that is being monitored at
the IC’s V
CC
pin. However, some systems may use the
open-drain output to level-shift from the monitored supply
to reset circuitry powered by some other supply (Figure
4). Note that as the SS8038L’s V
CC
decreases below 1V,
so does the IC’s ability to sink current at
RESET
. Also,
with any pull-up,
RESET
will be pulled high as
VCC
de-
cays toward 0. The voltage where this occurs depends
on the pull-up resistor value and the voltage to which it is
connected.
Benefits of Highly Accurate Reset Threshold
Most µP supervisor ICs have reset threshold voltages
between 5% and 10% below the value of nominal supply
voltages. This ensures a reset will not occur within 5% of
the nominal supply, but will occur when the supply is 10%
below nominal.
When using ICs rated at only the nominal supply ±5%,
this leaves a zone of uncertainty where the supply is be-
tween 5% and 10% low, and where the reset may or
may not be asserted.
The SS803xx463/803xx308 use highly accurate circuitry
to ensure that reset is asserted close to the 5% limit, and
long before the supply has declined to 10% below nomi-
nal.
GND
GND
Figure 3. Interfacing to µPs with bidirectional
reset I/O
+5.0V
+3.3V
V
CC
R
PULL-UP
V
CC
8038
RESET
5V SYSTEM
RESET
INPUT
GND
GND
Figure 4. SS8038L open-drain
RESET
output
allows use with multiple supplies
10/6/2004 Rev.2.20
www.SiliconStandard.com
8 of 9
SSC [ SILICON STANDARD CORP. ]
