NXP Semiconductors MKW01Z128, Reference Manual, Page: 802 / 884

Because the clocks are halted, the UART module resumes operation upon exit from stop,
only in stop and VLPS mode. Software must ensure stop mode is not entered while there
is a character (including preamble, break and normal data) being transmitted out of or
received into the UART module, that means UART_S1[TC] =1, UART_S1[TDRE] = 1,
and UART_S2[RAF] = 0 must all meet before entering stop mode.
33.4.7.3 Loop mode
When UART_C1[LOOPS] is set, the UART_C1[RSRC] bit in the same register chooses
between loop mode (UART_C1[RSRC] = 0) or single-wire mode (UART_C1[RSRC] =
1). Loop mode is sometimes used to check software, independent of connections in the
external system, to help isolate system problems. In this 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.
33.4.7.4 Single-wire operation
When UART_C1[LOOPS] is set, UART_C1[RSRC] 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. In single-wire 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.
Chapter 33 Universal asynchronous receiver/transmitter (UART1 and UART2)
MKW01Z128 MCU Reference Manual, Rev. 3, 04/2016
Freescale Semiconductor, Inc. 671