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Rev. 1.00
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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
UART Serial Interfaces – UART0 and UART1
Mode B
Mode B is an integrated full-duplex asynchronous serial communication interface. The 8 bits
of data are received via the RXD1 pin while the TXD1 pin provides the data transmit for this
communication. The data will then be transferred to the Transmit Shift Register from where it
will be shifted out onto the TXD1 pin by the UART1 Baud rate generator. The internal Baud rate
generator is enabled/disabled by the BD1 bit of the SBRCON register. The S1RELL/S1RELH
registers must be used to setup the Baud rate generator.
Data transmission is started by writing data to the S1BUF register. The TXD1 pin outputs data.
The first bit transmitted is a start bit, always “0”, then 8 bits of data, after which a stop bit, always “1”,
is transmitted.
Data to be received by the UART1 is accepted on the external RXD1 pin. When reception starts,
UART1 synchronises with the falling edge detected by the RXD1 pin. Input data is available after
one byte of data is complete in the S1BUF register and the value of the STOP bit is available as the
RB81 flag in the S1CON register. During the reception process, the S1BUF data and the RB81 bit
will remain unchanged until the process is complete.
UART1 Mode B Timing Diagram