M68HC11
REFERENCE MANUAL
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
MOTOROLA
9-19
output buffer is forced on. When the transmitter is disabled and the transmission in
progress is completed, the pin reverts to being a general-purpose I/O pin controlled by
the PORTD and DDRD registers. In some systems, it is very important to know just
what will happen to the pin when the SCI stops controlling it. Another feature of this
pin buffer allows it to act like an open-collector-type buffer, which helps prevent other-
wise catastrophic problems if two or more drivers connected to this same TxD line
were to become enabled at the same time. Finally, the DDRD1 control bit still influenc-
es what is returned on reads of PORTD even though it is overridden in terms of con-
trolling the output buffer enable.
Figure 9-3
is a simplified block diagram of the TxD pin logic that illustrates the points
of the following discussion. The MOS transistor-level schematic and a more detailed
description of the TxD pin logic are found in
7.3.6 Port D
.
Figure 9-3 TxD Pin Logic Block Diagram
Output buffer [1] is enabled by DDRD or by the SCI transmitter on (XMITON) signal.
The XMITON signal is active while the TE bit is one and long enough after TE is written
to zero to allow any currently transmitting character to finish. The DWOM signal deter-
mines whether port D pins will act as open-collector-type drivers or as totem-pole-type
drivers. DWOM simultaneously controls all six port D pins; thus, if open-collector-type
operation is needed on any port D pin(s), it must be used on all six. A fairly common
case might be where the SPI needed open-collector operation so DWOM is set to one.
In this case, the TxD pin needs a pull-up resistor since the internal high-side driver was
disabled.
The data driven out the TxD pin is selected by multiplexer [2]. When the transmitter is
enabled, SCI transmit data will drive the pin buffer. When the transmitter is disabled,
D
C
Q
Q
HFF
OUTPUT
BUFFER
DWOM
PD1
TxD
MUX
A
B
A/B
MUX
A
B
A/B
INPUT
BUFFER
[2]
[4]
[6]
[1]
[3]
XMITON
WPORTD
DDRD1
INTERNAL
DATA BUS
RPORTD
SCI
TRANSMIT
DATA
ENABLE
OPEN DRAIN/TOTEM
[5]