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CT—Clear-to-Send Lost
This status bit indicates that the CTS signal was negated during transmission of this char-
acter. If this occurs, the CTS bit in the UART event register will also be set.
If the CTS signal was negated during transmission, and the CP
transmits this character in the middle of buffer transmission, the
CTS signal could actually have been negated either during this
character's transmission (i.e., CHARACTER8) or during a buffer
character's transmission. In this case, the CP sets the CT bit
both here and in the Tx BD status word.
A—Address
When working in a multidrop configuration, the user should include the address bit in this
position.
CHARACTER8—Flow Control Character Value
Any 7- or 8-bit character value may be transmitted. This value may be modified only while
the REA bit is cleared. A 7-bit character should comprise bits 6–0.
4.5.11.8 Send Break
A break is an all-zeros character without stop bits—i.e., 9 to 13 continuous zeros. A break
is sent by issuing the STOP TRANSMIT command. The UART completes transmission of
any outstanding data in the FIFO and then sends 9 to 13 zeros (depending on the UM1–
UM0, SL, PEN, and CL bits in the UART mode register). The UART transmits a program-
mable number of break characters according to the value of the break count register
(BRKCR), and then reverts to idle or sends data if the RESTART TRANSMIT command was
given before completion. Upon transmission of the entire set of break characters, the trans-
mitter sends at least one high bit before transmitting any data to guarantee recognition of a
valid start bit.
4.5.11.9 Send Preamble (IDLE)
A preamble sequence gives the programmer a convenient way of ensuring that the line goes
idle before starting a new message. The preamble sequence length is 9 to 13 consecutive
ones (depending on the UM1–UM0, SL, PEN, and CL bits in the UART mode register). If the
preamble bit in a BD is set, the SCC will send a preamble sequence before transmitting that
data buffer.
4.5.11.10 Wakeup Timer
By issuing the ENTER HUNT MODE command, the user can temporarily disable the UART
receiver. It will remain inactive until an idle or address character is recognized (depending
on the setting of UM1–UM0).
If the UART is still in the process of receiving a message that the user has already decided
to discard, the message may be aborted by issuing the ENTER HUNT MODE command.
The UART receiver will be re-enabled when the message is finished by detecting one idle
MOTOROLA
NOTE
MC68302 USER'S MANUAL
Communications Processor (CP)
4-53
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