Datasheet

Functional Description
MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor 167
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.
14.5.6 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.
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.
TXD
RXD
WOMS
DDRS BIT = 1
DDRS BIT = 0
GENERAL-
PURPOSE I/O
TXD
RXD
WOMS
GENERAL-
PURPOSE I/O
H
TRANSMITTER
RECEIVER
TRANSMITTER
RECEIVER