M68HC11
REFERENCE MANUAL
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
MOTOROLA
9-7
When the SCI transmitter is active, bit 1 of DDRD is overridden, and the corresponding
output buffer is forced on and is driven by SCI logic (as opposed to port output logic).
The transmitter is active (controlling the PD1/TxD pin) whenever the transmitter en-
able bit (TE in the SCCR2 register) is one or an unfinished character is being transmit-
ted after the TE bit is disabled. Writes to bit 1 of port D 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 transmitter later relinquishes control of the pin, the logic
value in this latch will drive the PD1/TxD pin. Although the DDRD1 bit does not affect
the pin while the SCI transmitter is active, it still affects what is returned when port D
is read. If DDRD1 is zero, the pin is read (reflects what the SCI transmitter is currently
driving out of the pin). If DDRD1 is one (suggesting the pin should be an output), the
value in the internal port D bit 1 latch is returned (reflects what the pin would revert to
if the SCI transmitter relinquishes control of the pin).
All six bits of port D are affected by the port D wired-OR mode control bit (DWOM in
the SPCR). Whenever DWOM is one, the high-side driver (P-channel device) for all
port D pins is disabled. This disabling makes port D pins behave somewhat like open-
collector outputs; thus, an external pull-up resistor is needed for any port D pin being
used as an output (general-purpose or peripheral subsystem outputs). The DWOM bit
does not affect the use of port D pins as inputs.
9.2.2 Baud-Rate Control Register (BAUD)
The following register and paragraphs describe the BAUD control register, which is
used to set the bit rate for the SCI system. Normally, this register is written once during
initialization to set the baud rate for SCI communications. Both the receiver and the
transmitter use the same baud rate, which is derived from the MCU bus rate clock. A
two-stage divider is used to develop customary baud rates from normal MCU crystal
frequencies; therefore, it is not necessary to use special baud-rate crystal frequencies.
Table 9-1
and
Table 9-2
should be adequate for most users, but a more comprehen-
sive tabulation of baud rates is provided in
Table 9-3
to help users with unusual re-
quirements.
TCLR — Clear Baud-Rate Timing Chain (Test Modes Only)
This bit is disabled and remains low in any mode other than test or bootstrap modes.
Reset clears this bit. While in test or bootstrap modes, writing a one to this bit causes
the baud-rate counter chains to be reset. Because the one state of this bit is transitory,
reads always return a logic zero. This control bit is intended only for factory testing of
the MCU.
SCP[1:0] — SCI Baud-Rate Prescale Selects
These two bits select a prescale factor for the SCI baud-rate generator. The output fre-
quency of this prescaler determines the highest available baud rate in the system. The
BAUD —
Baud Rate Control Register
$102D
BIT 7
TCLR
0
6
0
0
5
4
3
2
1
BIT 0
SCR0
U
SCP1
0
SCP0
0
RCKB
0
SCR2
U
SCR1
U
RESET: