Zilog Z80230 - Manual (Page 160)

SCC/ESCC
User Manual
UM010903-0515 Register Descriptions
153
These bits control the number of bits in each byte transferred to the transmit buffer. Bits sent must
be right justified with the least significant bits first.
The Five Or Less mode allows transmission of one to five bits per character. For five or fewer bits
per character, the data character must be formatted as shown below in Table 5-5. In the Six or
Seven Bits/Character modes, unused data bits are ignored.
Bit 4: Send Break control bit
When set, this bit forces the TxD output to send continuous 0s beginning with the following trans-
mit clock, regardless of any data being transmitted at the time. This bit functions whether or not
the transmitter is enabled. When reset, TxD continues to send the contents of the Transmit Shift
register, which might be syncs, data, or all 1s. If this bit is set while in the X21 mode (Monosync
and Loop mode selected) and character synchronization is achieved in the receiver, this bit is auto-
matically reset and the transmitter begins sending syncs or data. This bit is also reset by a channel
or hardware reset.
Bit 3: Transmit Enable
Data is not transmitted until this bit is set, and the TxD output sends continuous 1s unless Auto
Echo mode or SDLC Loop mode is selected. If this bit is reset after transmission starts, the trans-
mission of data or sync characters is completed. If the transmitter is disabled during the transmis-
sion of a CRC character, sync or flag characters are sent instead of CRC. This bit is reset by a
channel or hardware reset.
Transmit Bits per Character
Bit 7 Bit 6
0 0 5 or less bits/character
0 1 7 bits/character
1 0 6 bits/character
1 1 8 bits/character
Note: For five or less bits per character selection in
WR5, the following encoding is used in the data sent
to the transmitter. D is the data bit(s) to be sent.
D7 D6 D5 D4 D3 D2 D1 D0
1
1 1 1 0 0 0 D Sends one data bit
1 1 1 0 0 0 D D Sends two data bits
1 1 0 0 0 D D D Sends three data bits
1 0 0 0 D D D D Sends four data bits
0 0 0 D D D D D Sends five data bits