NXP Semiconductors K32 L2A Series, Reference Manual, Page: 839 / 1379

31.3.2.3 Transceiver driver enable
The transmitter can use LPUART_RTS as an enable signal for the driver of an external
transceiver. See
Transceiver driver enable using LPUART_RTS for details. If the
request-to-send operation is enabled, when a character is placed into an empty transmitter
data buffer, LPUART_RTS asserts one bit time before the start bit is transmitted.
LPUART_RTS remains asserted for the whole time that the transmitter data buffer has
any characters. LPUART_RTS deasserts one bit time after all characters in the
transmitter data buffer and shift register are completely sent, including the last stop bit.
Transmitting a break character also asserts LPUART_RTS, with the same assertion and
deassertion timing as having a character in the transmitter data buffer.
The transmitter's LPUART_RTS signal asserts only when the transmitter is enabled.
However, the transmitter's LPUART_RTS signal is unaffected by its LPUART_CTS
signal. LPUART_RTS will remain asserted until the transfer is completed, even if the
transmitter is disabled mid-way through a data transfer.
31.3.2.4 Transceiver driver enable using LPUART_RTS
RS-485 is a multiple drop communication protocol in which the LPUART transceiver's
driver is 3-stated unless the UART is driving. The LPUART_RTS signal can be used by
the transmitter to enable the driver of a transceiver. The polarity of LPUART_RTS can
be matched to the polarity of the transceiver's driver enable signal.
TRANSMITTER
UART
RECEIVER
DRIVER
RS-485 TRANSCEIVER
RECEIVER
TXD
RTS_B
RXD
DI
DE
RO
RE_B
Y
Z
A
B
Figure 31-4. Transceiver driver enable using LPUART_RTS
In the figure, the receiver enable signal is asserted. Another option for this connection is
to connect LPUART_RTS to both DE and RE_B. The transceiver's receiver is disabled
while driving. A pullup can pull LPUART_RX to a non-floating value during this time.
This option can be refined further by operating the LPUART in single wire mode, freeing
the LPUART_RX pin for other uses.
Chapter 31 Low Power Universal Asynchronous Receiver/Transmitter (LPUART)
K32 L2A Reference Manual, Rev. 2, 01/2020
NXP Semiconductors 839