Global Mixed-mode Technology Inc.
Interfacing to µPs with Bidirectional Reset Pins
Since the RESET output on the G685L is open drain,
this device interfaces easily with µPs that have bidirec-
tional reset pins, such as the Motorola 68HC11. Connect-
ing 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 (Figure 3).
G684/G685
+5.0V
+5.0
V
CC
R
PULL-UP
+3.3V
V
CC
G685
RESET
5V SYSTEM
RESET
INPUT
V
CC
GND
V
CC
R
PULL-UP
V
CC
GND
G685
RESET
µP
RESET MOTOROLA
INPUT
68HCXX
Figure 4. G685L Open-Drain
RESET
Output Allows
Use with Multiple Supplies
GND
GND
Figure 3. Interfacing to µPs with Bidirectional Reset
I/O
G685L Open-Drain
RESET
Output Allows Use with
Multiple Supplies
Generally, the pull-up connected to the G685L 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 sup-
ply to reset circuitry powered by some other supply
(Figure 4). Note that as the G685L’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 µP supervisor ICs have reset threshold voltages
between 5% and 10% below the value of nominal sup-
ply voltages. This ensures a reset will not occur within
5% of the nominal supply, but will occur when the sup-
ply 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 G68_ _463/G68_ _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.
Ver: 1.2
Jun 03, 2004
TEL: 886-3-5788833
http://www.gmt.com.tw
6
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
