Global Mixed-mode Technology Inc.
V
CC
G670/G671
V
CC
R
PULL-UP
V
CC
G671
RESET
µP
RESET
MOTOROLA
INPUT
68HCXX
Interfacing to
µ
Ps with Bidirectional Reset Pins
Since the RESET output on the G671L is open drain,
this device interfaces easily with
µ
Ps that have bidirec-
tional reset pins, such as the Motorola 68HC11. Con-
necting the
µ
P supervisor
’
s RESET output directly to
the microcontroller’s (
µ
C
’
s) RESET pin with a single
pull-up resistor allows either device to assert reset
(Figure 3).
G671L Open-Drain
RESET
Output Allows Use with
Multiple Supplies
Generally, the pull-up connected to the G671L 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 G671L
’
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
decays 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
µ
P 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
between 5% and 10% low, and where the reset may or
may not be asserted.
The G67_ _463/G67_ _308 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
nominal.
GND
GND
Figure 3. Interfacing to
µ
Ps with Bidirectional Re-
set I/O
+5.0V
+3.3V
V
CC
R
PULL-UP
V
CC
G671
RESET
5V SYSTEM
RESET
INPUT
GND
GND
Figure 4. G671L Open-Drain
RESET
Output Allows
Use with Multiple Supplies
Ver: 1.5
Dec 03, 2005
TEL: 886-3-5788833
http://www.gmt.com.tw
10
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
