
M68HC11
REFERENCE MANUAL
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
MOTOROLA
9-15
synchronized to this bit-rate clock, there will be an uncertainty about exactly when a
character will start being transmitted relative to when it was written to the TDR. The
transmit bit-rate clock is free running as opposed to being started when a character is
written to the TDR.
Transmitter logic adds a zero start bit and a one stop bit to the data characters pre-
sented by the CPU for transmission. The transmitter can be configured to send char-
acters with eight (M = 0) or nine (M = 1) data bits. When the TDR is able to accept a
new data character, the TDRE status flag is set, and an interrupt can optionally be gen-
erated. Another status flag (TC) and optional interrupt are produced when the trans-
mitter has finished sending everything in its queue. In addition to data characters, the
transmitter is capable of sending idle-line characters and break characters, which are
useful in multi-drop SCI networks. The transmitter is double buffered, which means
that one character can be in the parallel TDR while another is in the transmit shift reg-
ister being sent. In the case of queued idle and break characters, three characters can
be in the queue, but no more than two can be data characters. The last topic of dis-
cussion for the transmitter will be the TxD pin buffer. This flexible buffer can be used
to control what happens to the TxD pin when the transmitter is finished using the pin.
9.3.1 Eight- and Nine-Bit Data Modes
The M bit in the SCCR1 determines the length of SCI characters for both the transmit-
ter and receiver. The most common configuration is one start bit, eight data bits, and
one stop bit, which is selected by M equals zero. When M equals one, characters are
composed of one start bit, nine data bits, and one stop bit. In this 9-bit data mode, the
low-order eight bits come from the normal TDR, and the ninth bit comes from the T8
bit of SCCR1. Data is transmitted LSB first, and this ninth bit becomes the new MSB,
which is transmitted just before the stop bit. Since this bit is adjacent to the stop bit, it
can be used as an extra stop bit by setting T8 to one.
Another common use for the 9-bit data format is in conjunction with the address-mark
variation of receiver wake up. In a multi-drop SCI network, all receivers evaluate the
first characters of a message to decide whether or not this message is important to this
receiver. If not, receiver wake up may be activated by writing a one to the RWU bit in
SCCR2. A one in RWU causes the receiver to ignore any other characters in the mes-
sage, thus allowing the MCU to perform more useful functions than responding to in-
terrupts from the SCI. The SCI receiver is still monitoring characters normally except
that status flags and interrupts are not being produced. When address-mark wake up
is being used, the SCI receive logic automatically clears RWU when it sees a character
whose MSB is one. The one in the MSB of a character indicates that this character is
the first addressing character of a new message; thus, all receivers should wake up
and evaluate this character. The 9-bit data format allows for full 8-bit data characters
to be used in the body of a message while the ninth bit acts as the address marker.
The first character of each message will have this ninth bit set to one; whereas, the
remaining characters in the message will have a zero in this ninth bit. Address-mark
wake up can be used with 8-bit data format, but message characters could only use
the lower seven bits for information.
The ninth bit can also be used as a parity bit. Mark or space parity can be produced