MCP131X datasheet(33/44 Pages) MICROCHIP

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DS21985B-page 33

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 control-

ler pin, an indeterminate logic level may result if there

is a logic conflict. This can be avoided by adding a

4.7 k

Ω resistor in series with the output of the

MCP131X/2X (Figure 5-7). If there are other compo-

nents 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 con-

nected 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 VDD = 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 VDD = 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 capaci-

tances and hold the output low (Figure 5-8).

Similarly for devices where the Reset signal is active-

high, a pull-up resistor to VDD is required to ensure a

valid high Reset signal for VDD 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

Ω will be suitable

for most applications).

FIGURE 5-8:

Ensuring a valid active-low

Reset pin output state as VDD approaches 0V.

MCP13XX

VDD

RST

GND

MCLR

GND

Buffered Reset

To Other System

Components

MCU

4.7 k

Buffer

PIC®

MCP13XX

100 k

RST

GND

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