NXP Semiconductors KL25 Series - Freescale Semiconductor, Inc

external system, to help isolate system problems. In this mode, the transmitter output is
internally connected to the receiver input and the RxD pin is not used by the UART, so it
reverts to a general-purpose port I/O pin.
40.3.6.4 Single-wire operation
When UART_C1[LOOPS] is set, the RSRC bit in the same register chooses between
loop mode (UART_C1[RSRC] = 0) or single-wire mode (UART_C1[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 UART_C3[TXDIR] bit controls the direction of serial data on
the TxD pin. When UART_C3[TXDIR] is cleared, the TxD pin is an input to the UART
receiver and the transmitter is temporarily disconnected from the TxD pin so an external
device can send serial data to the receiver. When UART_C3[TXDIR] is set, the TxD pin
is an output driven by the transmitter, the internal loop back connection is disabled, and
as a result the receiver can not receive characters that are sent out by the transmitter.
Functional description
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
770 Freescale Semiconductor, Inc.