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Chapter 46 Universal Asynchronous Receiver/Transmitter (UART) / FlexSCI
46.5.1.6 Hardware flow control
The transmitter supports hardware flow control by gating the transmission with the value
of CTS. If the clear-to-send operation is enabled, the character is transmitted when CTS
is asserted. If CTS is deasserted in the middle of a transmission with characters remaining
in the receiver data buffer, the character in the shift register is sent and TXD remains in
the mark state until CTS is reasserted.
If the clear-to-send operation is disabled, the transmitter ignores the state of CTS. Also, if
the transmitter is forced to send a continuous low condition because it is sending a break
character, the transmitter ignores the state of CTS regardless of whether the clear-to-send
operation is enabled.
The transmitter's CTS signal can also be enabled even if the same UART receiver's RTS
signal is disabled.
46.5.1.7 Transceiver driver enable
The transmitter can use RTS as an enable signal for the driver of an external transceiver.
See
Transceiver driver enable using RTS
for details. If the request-to-send operation is
enabled, when a character is placed into an empty transmitter data buffer, RTS asserts
one bit time before the start bit is transmitted. RTS remains asserted for the whole time
that the transmitter data buffer has any characters. 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 RTS, with the same assertion
and deassertion timing as having a character in the transmitter data buffer.
The transmitter's RTS signal asserts only when the transmitter is enabled. However, the
transmitter's RTS signal is unaffected by its CTS signal. RTS will remain asserted until
the transfer is completed, even if the transmitter is disabled mid-way through a data
transfer.
The following figure shows the functional timing information for the transmitter. Along
with the actual character itself, TXD shows the start bit. The stop bit is also indicated,
with a dashed line if necessary.
KV4x Reference Manual, Rev. 2, 02/2015
Preliminary
Freescale Semiconductor, Inc.
1299
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