MC9S12XS256 Datasheet, PDF (429/736 Pages) Freescale Semiconductor, Inc – HCS12 Microcontrollers

Serial Communication Interface (S12SCIV5)
Enable single-wire operation by setting the LOOPS bit and the receiver source bit, RSRC, in SCI control
register 1 (SCICR1). Setting the LOOPS bit disables the path from the RXD pin to the receiver. Setting
the RSRC bit connects the TXD pin to the receiver. Both the transmitter and receiver must be enabled
(TE = 1 and RE = 1).The TXDIR bit (SCISR2[1]) determines whether the TXD pin is going to be used as
an input (TXDIR = 0) or an output (TXDIR = 1) in this mode of operation.
NOTE
In single-wire operation data from the TXD pin is inverted if RXPOL is set.
14.4.8 Loop Operation
In loop operation the transmitter output goes to the receiver input. The RXD pin is disconnected from the
SCI.
Transmitter
TXD
Receiver
RXD
Figure 14-31. Loop Operation (LOOPS = 1, RSRC = 0)
Enable loop operation by setting the LOOPS bit and clearing the RSRC bit in SCI control register 1
(SCICR1). 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).
NOTE
In loop operation data from the transmitter is not recognized by the receiver
if RXPOL and TXPOL are not the same.
14.5 Initialization/Application Information
14.5.1 Reset Initialization
See Section 14.3.2, “Register Descriptions”.
14.5.2 Modes of Operation
14.5.2.1 Run Mode
Normal mode of operation.
To initialize a SCI transmission, see Section 14.4.5.2, “Character Transmission”.
S12XS Family Reference Manual, Rev. 1.09
Freescale Semiconductor
429