HD64570 Datasheet, PDF (213/469 Pages) Hitachi Semiconductor – Serial Communications Adaptor

5.3.3 Bit Synchronous Mode
In bit synchronous mode, communication is performed using flags added to frame boundaries.
This mode is specified with the PRTCL2−PRTCL0 bits of mode register 0 (MD0).
The message format for bit synchronous mode is shown in figure 5.26. The address (A) and
control and information (C and I) fields are configured in byte units and are sent to the receive
buffer. Except the frame check sequence (FCS) field, data is transmitted or received beginning
with the least significant bit. The FCS field data is transmitted and received beginning with the
most significant bit.
Residual bit frames cannot be transmitted. During reception, when residual bits exist at the end of
receive data, the valid bits (residual bits) in the last character are justified to the high-order bit
positions, generating undefined low-order bits. The undefined bits cannot be distinguished from
valid bits. When a residual bit frame is received, the status of the last character indicates both the
residual bit frame and end of receive frame. (This status is indicated by the EOM and RBIT bits
of status register 2 (ST2).)
In bit synchronous mode, frame boundaries can only be detected by the flag pattern "01111110."
To prevent the same data pattern as the flag, the SCA inserts or deletes zeros in the data strings.
This function is called "zero insertion/deletion."
During transmission, the transmitter constantly monitors data strings between the opening and
closing flags. If the transmitter detects five consecutive ones in a data string in the transmit buffer,
it adds a zero to the end before transmitting the data string on the TXD line. For example, if the
transmitter detects in the transmit buffer "11111111" and "11011111," it inserts one zero in each
data string to transmit "111011111" and "110011111," respectively.
During reception, if the receiver detects one zero after five consecutive ones in a data string
received from the RXD line, it deletes the zero from the data string before storing the data in the
receive buffer. If the receiver detects six or more consecutive one's, it regards the data as a flag or
abort frame and so performs the specified protocol without zero deletion.
Rev. 0, 07/98, page 197 of 453