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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
UART Serial Interfaces – UART0 and UART1
UART1 Operating Modes
UART1 provides two operational modes, selected by the SM bit in the S1CON register. There are
two asynchronous modes, offering different baud rates and functional options. The following table
illustrates the different operational mode list. When the transmit/receive data process is complete,
the transmit/receive interrupt will take place and the interrupt request bit, TI1 or RI1, will be set
high.
UART1 Operating Modes
SM bit
Mode
Mode Name
Baud Rate
Synchronisation
Start/Stop Bits
1
Mode B
8-bit UART
Va�iable
As�nch�ono�s
1 Sta�t� 1 Stop
0
Mode A
9-bit UART
Va�iable
As�nch�ono�s
1 Sta�t� 1 Stop
Mode A
Mode A is an integrated full-duplex asynchronous serial communication interface. The 9 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 to the S1BUF register. The TXD1 pin outputs the serial
data. The first bit transmitted is a start bit, always “0”, and then 9 bits of data where the 9th bit is
taken from the TB81 bit of the S1CON register, 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,
the 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 ninth bit is available as the RB81
bit 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 A Timing Diagram