Datasheet

 2005-2012 Microchip Technology Inc. DS21985D-page 37
MCP131X/2X
5.7 Controllers and Processors With
Bidirectional I/O Pins
Some microcontrollers have bidirectional Reset pins.
Depending on the current drive capability of the
controller pin, an indeterminate logic level may result if
there is a logic conflict. This can be avoided by adding
a 4.7 k resistor in series with the output of the
MCP131X/2X (Figure 5-7). If there are other
components in the system that require a Reset signal,
they should be buffered so as not to load the Reset line.
If the other components are required to follow the
Reset I/O of the microcontroller, the buffer should be
connected as shown with the solid line.
FIGURE 5-7: Interfacing the
MCP131X/2X Push-Pull Outputs to a
Bidirectional Reset I/O.
5.8 RESET Signal Integrity During
Power-Down
The MCP131X/2X Reset output is valid to V
DD
= 1.0V.
Below this 1.0V, the output becomes an "open circuit"
and does not sink or source current. This means
CMOS logic inputs to the microcontroller will be floating
at an undetermined voltage. Most digital systems are
completely shut down well above this voltage.
However, in situations where the Reset signal must be
maintained valid to V
DD
= 0V, external circuitry is
required.
For devices where the Reset signal is active-low, a pull-
down resistor must be connected from the
MCP131X/2X Reset pin(s) to ground to discharge stray
capacitances and hold the output low (Figure 5-8).
Similarly, for devices where the Reset signal is active-
high, a pull-up resistor to V
DD
is required to ensure a
valid high Reset signal for V
DD
below 1.0V.
This resistor value, though not critical, should be
chosen such that it does not appreciably load the Reset
pin(s) under normal operation (100 k will be suitable
for most applications).
FIGURE 5-8: Ensuring a Valid Active-low
Reset Pin Output State as V
DD
Approaches 0V.
MCP13XX
V
DD
RST
GND
MCLR
GND
Buffered Reset
To Other System
Components
MCU
4.7 k
Buffer
PIC
®
MCP13XX
V
DD
V
DD
R
1
100 k
RST
GND