
MOTOROLA
9-6
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
M68HC11
REFERENCE MANUAL
er subsystem enable controls. The RWU bit allows software to put the receiver to sleep
and hardware to automatically wake it up by clearing this bit. The send break SBK bit
allows software to generate break characters on the TxD line.
The SCSR contains two transmitter status flags and five receiver-related status flags.
The transmitter generates flags for TDRE and TC. The receiver generates flags for
RDRF, OR, idle-line detect (IDLE), a noise flag (NF), and a framing error (FE) indica-
tion.
The SCDR is actually two separate registers. TDR is a write-only transmit data buffer
register, and RDR is a read-only receive data buffer register. When software reads
SCDR, it is accessing RDR; when software writes to SCDR, it is accessing TDR.
9.2.1 Port D Related Registers and Control Bits (PORTD, DDRD, SPCR)
The following registers are the port D related registers. Because the SCI system uses
the two LSBs of this port, only the interactions between general-purpose I/O and the
use of these pins by the SCI will be discussed. The actual MOS logic for port D pins is
shown and discussed in
7.3.6 Port D
.
Each internal peripheral subsystem interacts with port I/O pins in different ways. In
some cases, such as the SCI system, the internal subsystem overrides other pin con-
trols to actively take charge of the pin. In other cases, such as the SPI and pulse ac-
cumulator, the pin controls (data direction and others) still influence the configuration
of the pin logic. The user must never assume that all pins in a port are affected in the
same way by data direction controls.
When the SCI receiver is enabled (by the RE bit in the SCCR2 register), bit 0 of DDRD
is overridden, and the output buffer is disabled. Writes to port D bit 0 while the SCI has
control of the pin do not alter the logic state at the pin; however, any value written is
remembered in an internal latch. If the SCI receiver later relinquishes control of the pin,
the logic value in this latch will drive the PD0/RxD pin. Although the DDRD0 bit does
not affect the pin while the SCI receiver is enabled, it still affects what is returned when
port D is read. If DDRD0 is zero, the pin is read. If DDRD0 is one (suggesting the pin
should be an output), the value in the internal port D bit 0 latch is returned.
Bit 7
6
5
4
3
2
1
Bit 0
RESET:
REFER-
ENCE:
0
0
0
0
0
0
0
0
—
—
PD5/SS
PD4/SCK
PD3/
MOSI
PD2/
MISO
PD1/TxD
PD0/RxD
$1009
RESET:
0
0
0
0
DDRD5
0
DDRD4
0
DDRD3
0
DDRD2
0
DDRD1
0
DDRD0
0
DDRD
$1028
RESET:
SPIE
0
SPE
0
DWOM
0
MSTR
0
CPOL
0
CPHA
1
SPR1
U
SPR0
U
SPCR