MC9S08GB60 Datasheet, PDF (177/290 Pages) Motorola, Inc – Microcontrollers

SCI Registers and Control Bits
Note, because the clocks are halted, the SCI module will resume 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.
11.9.1 Loop Mode
When LOOPS = 1, 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 RxD1 pin is not used by the SCI, so it reverts to a
general-purpose port I/O pin.
11.9.2 Single-Wire Operation
When LOOPS = 1, the RSRC bit in the same register chooses between loop mode (RSRC = 0) or
single-wire mode (RSRC = 1). Single-wire mode is used to implement a half-duplex serial connection.
The receiver is internally connected to the transmitter output and to the TxD1 pin. The RxD1 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 TxD1 pin. When
TXDIR = 0, the TxD1 pin is an input to the SCI receiver and the transmitter is temporarily disconnected
from the TxD1 pin so an external device can send serial data to the receiver. When TXDIR = 1, the TxD1
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.
11.10 SCI Registers and Control Bits
The SCI has eight 8-bit registers to control baud rate, select SCI options, report SCI status, and for
transmit/receive data.
Refer to the direct-page register summary in the Memory section of this data sheet for the absolute address
assignments for all SCI registers. This section refers to registers and control bits only by their names. A
Freescale-provided equate or header file is used to translate these names into the appropriate absolute
addresses.
Some MCU systems have more than one SCI, so register names include placeholder characters to identify
which SCI is being referenced. For example, SCIxC1 refers to the SCIx control register 1 and SCI2C1 is
the control register 1 for SCI2.
11.10.1 SCI x Baud Rate Registers (SCIxBDH, SCIxBDL)
This pair of registers controls the prescale divisor for SCI baud rate generation. To update the 13-bit baud
rate setting [SBR12:SBR0], first write to SCIxBDH to buffer the high half of the new value and then write
to SCIxBDL. The working value in SCIxBDH does not change until SCIxBDL is written.
SCIxBDL is reset to a non-zero value, so after reset the baud rate generator remains disabled until the first
time the receiver or transmitter is enabled (RE or TE bits in SCIxC2 are written to 1).
MC9S08GB/GT Data Sheet, Rev. 2.3
Freescale Semiconductor
177