MC68HC812A4 Datasheet, PDF (248/342 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)
14.6.6 Loop Operation
In loop operation, the transmitter output goes to the receiver input. The
RXD pin is disconnected from the SCI and is available as a
general-purpose I/O pin.
Setting the data direction bit for the TXD pin connects the transmitter
output to the TXD pin. Clearing the data direction bit disconnects the
transmitter output from the TXD pin.
DDRS BIT = 1
TRANSMITTER
RECEIVER
WOMS
TXD
GENERAL-
RXD _{PURPOSE I/O}
H
TRANSMITTER
TXD
DDRS BIT = 0
RECEIVER
WOMS
GENERAL-
RXD _{PURPOSE I/O}
Figure 14-16. Loop Operation (LOOP = 1 and RSRC = 0)
Enable loop operation by setting the LOOPS bit and clearing the RSRC
bit in SCI control register 1 (SCCR1). Setting the LOOPS bit disables the
path from the RXD pin to the receiver. Clearing the RSRC bit connects
the transmitter output to the receiver input. 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 loop operation.
14.7 Register Descriptions and Reset Initialization
This section provides register descriptions and reset initialization.
MC68HC812A4 — Rev. 3.0
248
Serial Communications Interface Module (SCI)
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