
10-10
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Functional Description
10
10.6.3.3 Break Characters
Writing a logic one to the send break bit SBK in the SCI Control Register (SCICR) loads
the Transmit Shift Register with a break character. A break character contains all logic
zeros and has no start, stop, or parity bit. Break character length depends on the M bit in
the SCI Control Register (SCICR). As long as SBK is at logic one, transmitter logic
continuously loads break characters into the Transmit Shift Register. After software clears
the SBK bit, the shift register finishes transmitting the last break character and then
transmits at least one logic one. The automatic logic one at the end of the last break
character guarantees the recognition of the start bit of the next frame.
The SCI recognizes a break character when a start bit is followed by eight or nine logic
zero data bits and a logic zero where the stop bit should be. Receiving a break character
has these effects on SCI registers:
Sets the Framing Error flag (FE)
Sets the Receive Data Register Full flag (RDRF)
Clears the SCI Data Register (SCIDR)
May set the Overrun (OR) flag, Noise Flag (NF), Parity Error (PE) flag, or the
Receiver Active Flag (RAF). Please see the SCI Status Register in Section 10.9.4. 10.6.3.4 Preambles
A preamble contains all logic ones and has no Start, Stop, or Parity bit. A preamble length
depends on the M bit in the SCI Control Register (SCICR). The preamble is a
synchronizing mechanism that begins the first transmission initiated after modifying the
TE bit from 0 to 1.
If the TE bit is cleared during a transmission, the TXD pin becomes idle after completion
of the transmission in progress. Clearing and then setting the TE bit during a transmission
queues a preamble to be sent after the frame currently being transmitted.
Note:
Toggle the TE bit for a queued preamble when the TDRE flag becomes set and
immediately before writing the next character to the SCI Data Register.
When queueing a preamble, return the TE bit to logic one before the stop bit of
the current frame shifts out to the TXD pin. Setting TE after the stop bit appears
on TXD causes data previously written to the SCI Data Register to be lost.
10.6.3.5 Receiver
Figure 10-4 explains the block diagram of the SCI receiver with detailed discussion of the
receiver function in the following paragraphs.