M68HC11
REFERENCE MANUAL
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
MOTOROLA
9-11
as the last character from the previous message finishes being shifted out. Finally,
write the first character of the next message to the SCDR. This new character will start
transmitting as soon as the queued idle character finishes.
RE — Receive Enable
0 = SCI receiver disabled
1 = SCI receiver enabled
While the SCI receiver is disabled, the RDRF, IDLE, OR, NF, and FE status flags can-
not become set. If these flags were set, turning off RE does not cause them to be
cleared.
RWU — Receiver Wake Up
0 = Normal SCI receiver operation (wake-up feature not enabled).
1 = Places the SCI receiver in a standby mode where receiver-related interrupts
are inhibited until some hardware condition is met to wake up the sleeping re-
ceiver.
The condition that wakes the receiver up depends on which method of wake up was
specified with the WAKE bit in SCCR1.
Although it is possible for software to write the RWU bit to zero, is very unusual to do
so. The normal sequence is for software to set the RWU bit after deciding that a par-
ticular SCI message is of no interest. Setting the RWU bit causes the receiver to go to
sleep (ignore further receiver interrupt sources) until the start of the next message. Re-
ceiver wake-up logic recognizes when the unimportant message is over and automat-
ically clears the RWU bit to wake up the sleeping receiver.
SBK — Send Break
0 = Normal transmitter operation.
1 = Enable transmitter to send synchronous break characters.
Whenever the SBK bit is written to one, at least one character time of break will be
queued and sent. In the context of the M68HC11, a break character causes the TxD
line to go to logic zero for 10 (11 if M = 1) bit times.
In old teletype systems, a break was caused by simply disconnecting the serial line,
which caused the line to go to logic zero for some asynchronous length of time (usually
as long as the break key was pressed). A receiver seeing a break character produced
by an M68HC11 would receive an all-zero character with a framing error (FE) because
the line would be low where the receiver expected to see a logic-high stop bit.
The break characters in the M68HC11 are synchronous because no partial character
times of the break condition are ever produced. When SBK is set to one, a break char-
acter is queued pending completion of any character currently shifting out of the trans-
mit shift register. When the transmit shift register becomes available, the queued
break character is jammed into the shift register to be serially sent, and, if the SBK bit
is still one, another break is queued. It will always be at least one character time from
when the SBK bit is written back to zero before the transmitter can resume sending
normal characters.
9.2.5 SCI Status Register (SCSR)
The seven status bits associated with the SCI system are located in the SCSR, which
is depicted in the following register. Some of these bits optionally generate hardware