C8051F040/1/2/3/4/5/6/7
Rev. 1.5 269
21.1.3. Mode 2: 9-Bit UART, Fixed Baud Rate
Mode 2 provides asynchronous, full-duplex communication using a total of eleven bits per data byte: a start
bit, 8 data bits (LSB first), a programmable ninth data bit, and a stop bit. Mode 2 supports multiprocessor
communications and hardware address recognition (see Section 21.2). On transmit, the ninth data bit is
determined by the value in TB80 (SCON0.3). It can be assigned the value of the parity flag P in the PSW or
used in multiprocessor communications. On receive, the ninth data bit goes into RB80 (SCON0.2) and the
stop bit is ignored.
Data transmission begins when an instruction writes a data byte to the SBUF0 register. The TI0 Transmit
Interrupt Flag (SCON0.1) is set at the end of the transmission (the beginning of the stop-bit time). Data
reception can begin any time after the REN0 Receive Enable bit (SCON0.4) is set to logic 1. After the stop
bit is received, the data byte will be loaded into the SBUF0 receive register if RI0 is logic 0 and one of the
following requirements are met:
• SM20 is logic 0
• SM20 is logic 1, the received 9th bit is logic 1,
and the received address matches the UART0 address
as described in Section 21.2.
If the above conditions are satisfied, the eight bits of dat
a are stored in SBUF0, the ninth bit is stored in
RB80 and the RI0 flag is set. If these conditions are not met, SBUF0 and RB80 will not be loaded and the
RI0 flag will not be set. An interrupt will occur if enabled when either TI0 or RI0 are set.
The baud rate in Mode 2 is either SYSCLK / 32 or SYSCLK / 64, according to the value of the SMOD0 bit
in register SSTA0.
Equation 21.5. Mode 2 Baud Rate
Figure 21.5. UART0 Modes 2 and 3 T
iming Diagram
Figure 21.6. UART0 Modes 1, 2, and 3 Interconnect Diagram
BaudRate 2
SMOD0
SYSCLK
64
----------------------
=
D1D0 D2 D3 D4 D5 D6 D7
START
BIT
MARK
STOP
BIT
BIT TIMES
BIT SAMPLING
SPACE
D8
OR
RS-232
C8051Fxxx
RS-232
LEVEL
XLTR
TX
RX
C8051Fxxx
RX
TX
MCU
RX
TX
