Datasheet

In single-wire mode, the SCI_C3[TXDIR] bit controls the direction of serial data on the
TxD pin. When SCI_C3[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 SCI_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
MC9S08PA60 Reference Manual, Rev. 1, 9/2012
424 Freescale Semiconductor, Inc.