NXP Semiconductors Kinetis KE1xZ256, Reference Manual

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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 RTS_B, with
the same assertion and deassertion timing as having a character in the transmitter data
buffer.
The transmitter's RTS_B signal asserts only when the transmitter is enabled. However,
the transmitter's RTS_B signal is unaffected by its CTS_B signal. RTS_B will remain
asserted until the transfer is completed, even if the transmitter is disabled mid-way
through a data transfer.
46.4.2.4 Transceiver driver enable using RTS_B
RS-485 is a multiple drop communication protocol in which the LPUART transceiver's
driver is 3-stated unless the LPUART is driving. The RTS_B signal can be used by the
transmitter to enable the driver of a transceiver. The polarity of RTS_B 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 46-4. Transceiver driver enable using RTS_B
In the figure, the receiver enable signal is asserted. Another option for this connection is
to connect RTS_B to both DE and RE_B. The transceiver's receiver is disabled while
driving. A pullup can pull RXD to a non-floating value during this time. This option can
be refined further by operating the LPUART in single wire mode, freeing the RXD pin
for other uses.
46.4.3 Receiver functional description
In this section, the receiver block diagram is a guide for the overall receiver functional
description. Next, the data sampling technique used to reconstruct receiver data is
described in more detail. Finally, different variations of the receiver wakeup function are
explained.
Chapter 46 Low Power Universal Asynchronous Receiver/Transmitter (LPUART)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors 1209