MC68HC812A4 Datasheet, PDF (247/342 Pages) Motorola, Inc – 16-bit device composed of standard on-chip peripheral modules connected by an intermodule bus. Modules include

Serial Communications Interface Module (SCI)
Functional Description
14.6.5 Single-Wire Operation
Normally, the SCI uses two pins for transmitting and receiving. In
single-wire operation, the RXD pin is disconnected from the SCI and is
available as a general-purpose I/O pin. The SCI uses the TXD pin for
both receiving and transmitting.
Setting the data direction bit for the TXD pin configures TXD as the
output for transmitted data. Clearing the data direction bit configures
TXD as the input for received data.
DDRS BIT = 1
TRANSMITTER
RECEIVER
WOMS
TXD
GENERAL-
RXD _{PURPOSE I/O}
TRANSMITTER
NC
DDRS BIT = 0
RECEIVER
TXD
GENERAL-
RXD _{PURPOSE I/O}
Figure 14-15. Single-Wire Operation (LOOPS = 1 and RSRC = 1)
Enable single-wire operation by setting the LOOPS bit and the receiver
source bit, RSRC, in SCI control register 1 (SCCR1). Setting the LOOPS
bit disables the path from the RXD pin to the receiver. Setting the RSRC
bit connects the receiver input to the output of the TXD pin driver. Both
the transmitter and receiver must be enabled (TE = 1 and RE = 1).
The wired-OR mode select bit, WOMS, configures the TXD pin for full
CMOS drive or for open-drain drive. WOMS controls the TXD pin in both
normal operation and in single-wire operation. When WOMS is set, the
data direction bit for the TXD pin does not have to be cleared for TXD to
receive data.
Advance Information
MOTOROLA
Serial Communications Interface Module (SCI)
MC68HC812A4 — Rev. 3.0
247