Datasheet

© 2005-2012 Microchip Technology Inc. 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 Tabl e 1-1.
The FAULT
/TXE also signals a mismatch between the
TXD input and the L
BUS 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 pin has an internal pull-up resistor of
approximately 750 kΩ.
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 L
BUS output driver.
2: FAULT
/TXE is true (0) when VREG not OK
and has disabled the L
BUS output driver.
TXD
In
RXD
Out
LINBUS
I/O
Thermal
Override
FAULT/
TXE
Definition
External
Input
Driven
Output
LHV
BB OFF H L FAULT, TXD driven low, LINBUS shorted to
V
BB (Note 1)
HHV
BB OFF H H OK
LLGND OFF H HOK
HLGND OFF H HOK, data is being received from the LIN
BUS
xxVBB ON H L FAULT, transceiver in thermal shutdown
xxV
BB xLxNO 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.