22.5.6.3 Loop modeWhen SCI_C1[LOOPS] is set, the SCI_C1[RSRC] bit in the same register choosesbetween loop mode (SCI_C1[RSRC] = 0) or single-wire mode (SCI_C1[RSRC] = 1).Loop mode is sometimes used to check software, independent of connections in theexternal system, to help isolate system problems. In this mode, the internal loop backconnection from the transmitter to the receiver causes the receiver to receive charactersthat are sent out by the transmitter.22.5.6.4 Single-wire operationWhen SCI_C1[LOOPS] is set, SCI_C1[RSRC] chooses between loop mode(SCI_C1[RSRC] = 0) or single-wire mode (SCI_C1[RSRC] = 1). Single-wire modeimplements a half-duplex serial connection. The receiver is internally connected to thetransmitter 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 SCI_C3[TXDIR] bit controls the direction of serial data on theTxD pin. When SCI_C3[TXDIR] is cleared, the TxD pin is an input to the SCI receiverand the transmitter is temporarily disconnected from the TxD pin so an external devicecan send serial data to the receiver. When SCI_C3[TXDIR] is set, the TxD pin is anoutput driven by the transmitter. In single-wire mode, the transmitter output is internallyconnected to the receiver input and the RxD pin is not used by the SCI, so it reverts to ageneral-purpose port I/O pin.Functional descriptionMC9S08SU16 Reference Manual, Rev. 5, 4/2017420 NXP Semiconductors
