Zilog Z80230 - Manual (Page 108)

SCC/ESCC
User Manual
UM010903-0515 Data Communication Modes
101
In character-oriented modes, a special bit pattern is used to provide character synchronization. The
SCC offers several options to support Synchronous mode including various sync generation and
checking, CRC generation and checking, as well as modem controls and a transmitter to receiver
synchronization function.
The number of bits per transmitted character is controlled by D6 and D5 of WR5 plus the way the
data is formatted within the transmit buffer. The bits in WR5 select the option of five, six, seven,
or eight bits per character. In all cases, the data must be right-justified, with the unused bits being
ignored except in the case of five bits per character. When the five bits per character option is
selected, the data must be formatted before being written to the transmit buffer to allow transmis-
sion of from one to five bits per character. This formatting is listed in Table on page 94.
An additional bit, carrying parity information, may be automatically appended to every transmit-
ted character by setting bit D0 of WR4 to 1. This parity bit is sent in addition to the number of bits
specified in WR4 or by the data format. If this bit is set to 1, the transmitter sends even parity; if
set to 0, the transmitted parity is odd. Parity is not typically used in synchronous applications
because the CRC provides a more reliable method for detecting errors.
Either of two CRC polynomials are used in Synchronous modes, selected by bit D2 in WR5. If this
bit is set to 1, the CRC-16 polynomial is used and, if this bit is set to 0, the CRC-CCITT polyno-
mial is used. This bit controls the selection for both the transmitter and receiver. The initial state of
the generator and checker is controlled by bit D7 of WR10. When this bit is set to 1, both the gen-
erator and checker have an initial value of all ones; if this bit is set to 0, the initial values are all
zeros.
4 (=0) select monosync mode
5 (=0) (8-bit sync character)
4 (=1) select bisync mode
5 (=0) (16-bit sync character)
4 (=1) select external sync mode
5 (=1) (external sync signal required)
6 (=0) select 1x clock mode
7 (=0)
WR6 7-0 sync character (low byte)
WR7 7-0 sync character (high byte)
WR10 1 select sync character length
Registers Used in Character-Oriented Modes
Reg Bit No Description