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46.9.5.1 Ready-to-receive using RTS
To help to stop overrun of the receiver data buffer, the RTS signal can be used by the
receiver to indicate to another UART that it is ready to receive data. The other UART can
send the data when its CTS signal is asserted. This handshaking conforms to the
TIA-232-E standard. A transceiver is necessary if the required voltage levels of the
communication link do not match the voltage levels of the UART's RTS and CTS signals.
TRANSMITTER
UART
RECEIVER
TRANSMITTER
UART
RECEIVER
TXD
CTS_B
RXD
RTS_B
RXD
RTS_B
TXD
CTS_B
Figure 46-89. Ready-to-receive
The transmitter's CTS signal can be used for hardware flow control whether its RTS
signal is used for hardware flow control, transceiver driver enable, or not at all.
46.9.5.2 Transceiver driver enable using RTS
RS-485 is a multiple drop communication protocol in which the UART transceiver's
driver is 3-stated unless the UART is driving. The RTS signal can be used by the
transmitter to enable the driver of a transceiver. The polarity of RTS can be matched to
the polarity of the transceiver's driver enable signal. See the following figure.
TRANSMITTER
UART
RECEIVER
DRIVER
RS-485 TRANSCEIVER
RECEIVER
TXD
RTS_B
RXD
DI
DE
RO
RE_B
Y
Z
A
B
Figure 46-90. Transceiver driver enable using RTS
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 UART in single wire mode, freeing the RXD pin for
other uses.
Chapter 46 Universal Asynchronous Receiver/Transmitter (UART) / FlexSCI
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
1325
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