MCP2021 datasheet(13/52 Pages) MICROCHIP

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DS22018F-page 13

MCP2021/2/1P/2P

1.4.8

FAULT/TXE

Fault Detect Output and Transmitter Enable Input

bidirectional pin.

This pin is an open-drain output. Its state is defined as

shown in Table 1-2. The transmitter driver is disabled

whenever this pin is low (‘0’), either from an internal

fault condition or by external drive. This allows the

transmitter to be placed in an off state and still allow the

voltage regulator to operate. Refer to Table 1-1.

The FAULT/TXE also signals a mismatch between the

TXD input and the LBUS level. This can be used to

detect a bus contention. Since the bus exhibits a

propagation delay, the sampling of the internal

compare is debounced to eliminate false faults.

This

has

internal

pull-up

resistor

approximately 750 k

Ω.

The FAULT/TXE pin sampled at a rate faster than every

10 µs.

TABLE 1-2:

FAULT/TXE TRUTH TABLE

1.4.9

RESET

RESET is an open-drain output pin. This pin reflects an

internal signal that tracks the internal system voltage

has reached a valid, stable level.

As long as the internal voltage is valid, this pin will keep

high impedance. When the system voltage drops below

the minimum required, the voltage regulator will shut

down and immediately convert the RESET output to

short to GND. A pull-up resistor is needed to change

the output to high/low voltage. When connected to a

micro-controller input, this can provide a warning that

the voltage regulator is shutting down (see Figure 1-2).

Alternately, it can act as an external brown-out by con-

necting the RESET output to MCLR (see Figure 1-2). In

addition to monitoring the internal voltage, RESET is

asserted immediately upon entering the Power-Down

mode.

Note 1: The FAULT/TXE pin is true (0) whenever

the internal circuits have detected a short

or thermal excursion and have disabled

the LBUS output driver.

2: FAULT/TXE is true (0) when VREG not OK

and has disabled the LBUS output driver.

TXD

RXD

Out

LINBUS

I/O

Thermal

Override

FAULT/TXE

Definition

External

Input

Driven

Output

VBB

OFF

FAULT, TXD driven low, LINBUS shorted to

VBB (Note 1)

VBB

OFF

GND

OFF

GND

OFF

OK, data is being received from the LINBUS

VBB

FAULT, transceiver in thermal shutdown

VBB

NO FAULT, the CPU is commanding the

transceiver to turn off the transmitter driver

Legend: x = don’t care

Note 1:

The FAULT/TXE is valid after approximately 25 µs after TXD falling edge. This is to eliminate false fault

reporting during bus propagation delays.

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