Global Mixed-mode Technology Inc.
G690/G691
Interfacing to µPs with Bidirectional Reset Pins
VCC
Since the RESET output on the G691L is open drain,
this device interfaces easily with µPs that have bidi-
rectional reset pins, such as the Motorola 68HC11.
VCC
VCC
RPULL-UP
Connecting the µP supervisor’s RESET output di-
µP
G691
rectly to the microcontroller’s (µC’s) RESET pin with
a single pull-up resistor allows either device to assert
reset (Figure 3).
RESET
INPUT
MOTOROLA
RESET
68HCXX
GND
GND
G691L Open-Drain RESET Output Allows Use with
Multiple Supplies
Generally, the pull-up connected to the G691L will
connect to the supply voltage that is being monitored
at the IC’s VCC 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 G691L’s VCC decreases
below 1V, so does the IC’s ability to sink current at
Figure 3. Interfacing to µPs with Bidirectional Reset
I/O
+5.0V
+3.3V
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.
VCC
VCC
RPULL-UP
5V SYSTEM
G691
RESET
RESET
INPUT
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.
GND
GND
Figure 4. G691L Open-Drain RESET Output Allows
Use with Multiple Supplies
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 G69_ _463/G69_ _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.
TEL: 886-3-5788833
http://www.gmt.com.tw
Ver: 1.3
Oct 31, 2002
9
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
