MSC8144E Reference Manual, Rev. 3
20-22
Freescale
Semiconductor
UART
20.4 Modes of Operation
The following sections summarize the UART modes of operation.
20.4.1 Run Mode
Run mode is the normal mode of operation.
20.4.2 Single-Wire Operation
Normally, the UART (SCI) uses two signals for transmitting and receiving data. In single-wire
operation,
URXD
is disconnected from the UART and is available as a GPIO signal (see Chapter
22, GPIO). The UART uses
UTXD
for both receiving and transmitting data. Setting the data
direction bit for
UTXD
, SCIDDR[DDRTX], configures
UTXD
as the output for transmitted data.
Clearing the data direction bit, SCIDDR[DDRTX], disables the transmitter to drive
UTXD
.
Enable single-wire operation by setting the SCICR[LOOPS] bit and the receiver source bit,
SCICR[RSRC]. Setting the SCICR[LOOPS] bit disables the path from
URXD
to the receiver.
Setting the SCICR[RSRC] bit connects the receiver input to the output of
UTXD
. Both the
transmitter and receiver must be enabled (SCICR[TE] = 1 and SCICR[RE] = 1). You can
configure
UTXD
(see Chapter 22, GPIO) for full CMOS drive or for open-drain drive. The
configuration bit controls
UTXD
in both normal operation and single-wire operation. The
configuration bit also allows the
UTXD
outputs to be tied together in a multiple-transmitter
system, which allows the
UTXD
signals of nonactive transmitters to follow the logic level of an
active one. External pull-up resistors are necessary when using open-drain outputs.
Figure 20-19. Single-Wire Operation (LOOPS = 1, RSRC = 1)
UTXD
URXD
Transmitter
General-Purpose I/O
Receiver
SCIDDR[DDRTX]
GPIO
UART
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