Register Descriptions and Reset Initialization
MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor
169
14.6.2 SCI Control Register 1
Read: Anytime
Write: Anytime
LOOPS — Loop Select Bit
LOOPS enables loop operation. In loop operation the RXD pin is disconnected from the SCI, and the
transmitter output goes into the receiver input. Both the transmitter and the receiver must be enabled
to use the loop function.
1 = Loop operation enabled
0 = Normal operation enabled
The receiver input is determined by the RSRC bit. The transmitter output is controlled by the
associated DDRS bit.
If the data direction bit for the TXD pin is set and LOOPS = 1, the transmitter output appears on the
TXD pin. If the DDRS bit is clear and LOOPS = 1, the TXD pin is idle (high) if RSRC = 0 and
WOMS — Wired-OR Mode Select Bit
WOMS configures the TXD and RXD pins for open-drain operation. WOMS allows TXD pins to be tied
together in a multiple-transmitter system. Then the TXD pins of non-active transmitters follow the logic
level of an active 1. WOMS also affects the TXD and RXD pins when they are general-purpose outputs.
External pullup resistors are necessary on open-drain outputs.
1 = TXD and RXD pins, open-drain when outputs
0 = TXD and RXD pins, full CMOS drive capability
RSRC — Receiver Source Bit
When LOOPS = 1, the RSRC bit determines the internal feedback path for the receiver.
1 = Receiver input connected to TXD pin
0 = Receiver input internally connected to transmitter output
SCI0: $00C2
SCI1: $00CA
Bit 7
6
5
4321
Bit 0
Read:
LOOPS
WOMS
RSRC
M
WAKE
ILT
PE
PT
Write:
Reset:
000
00000
Figure 14-19. SCI Control Register 1 (SC0CR1 or SC1CR1)
Table 14-7. Loop Mode Functions
LOOPS
RSRC
DDRSx(1)
WOMS
Function of TXD Pin
0
X
Normal operation
10
0
X
Loop mode; transmitter output connected to receiver input
TXD pin disconnected
10
1
0
Loop mode; transmitter output connected to receiver input
TXD is CMOS output
10
1
Loop mode; transmitter output connected to receiver input
TXD is open-drain output