NXP Semiconductors MC9S08LG16, Reference Manual

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Chapter 11 Serial Communications Interface (S08SCIV4)
MC9S08LG32 MCU Series, Rev. 5
Freescale Semiconductor 277
13.3.5.2 Stop Mode Operation
During all stop modes, clocks to the SCI module are halted.
In stop2 mode, all SCI register data is lost and must be re-initialized upon recovery from this mode. No
SCI module registers are affected in stop3 mode.
The receive input active edge detect circuit remains active in stop3 mode, but not in stop2. An active edge
on the receive input brings the CPU out of stop3 mode if the interrupt is not masked (RXEDGIE = 1).
Because the clocks are halted, the SCI module resumes operation upon exit from stop (only in stop3 mode).
Software should ensure stop mode is not entered while there is a character being transmitted out of or
received into the SCI module.
13.3.5.3 Loop Mode
When LOOPS is set, the RSRC bit in the same register chooses between loop mode (RSRC = 0) or
single-wire mode (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 transmitter output is
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.
13.3.5.4 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.