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MCF51QE128 MCU Series Reference Manual, Rev. 3
Freescale Semiconductor 311
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internally connected to the receiver input and the RxD pin is not used by the SCI, so it reverts to a
general-purpose port I/O pin.
15.3.5.4 Single-Wire Operation
When LOOPS is set, the RSRC bit in the same register chooses between loop mode (RSRC = 0) or
single-wire mode (RSRC = 1). Single-wire mode implements a half-duplex serial connection. The receiver
is internally connected to the transmitter output and to the TxD pin. The RxD pin is not used and reverts
to a general-purpose port I/O pin.
In single-wire mode, the TXDIR bit in SCIxC3 controls the direction of serial data on the TxD pin. When
TXDIR is cleared, the TxD pin is an input to the SCI receiver and the transmitter is temporarily
disconnected from the TxD pin so an external device can send serial data to the receiver. When TXDIR is
set, the TxD pin is an output driven by the transmitter. In single-wire mode, the internal loop back
connection from the transmitter to the receiver causes the receiver to receive characters that are sent out
by the transmitter.