Rainbow Electronics AT89C51SND1C - Manual (Page 137)

137
AT8xC51SND1C
4109E–8051–06/03
Asynchronous Modes
(Modes 1, 2 and 3) The Serial Port has one 8-bit and 2 9-bit asynchronous modes of operation. Figure 101
shows the Serial Port block diagram in such asynchronous modes.
Figure 101. Serial I/O Port Block Diagram (Modes 1, 2 and 3)
Mode 1 Mode 1 is a full-duplex, asynchronous mode. The data frame (see Figure 102) consists
of 10 bits: one start, eight data bits and one stop bit. Serial data is transmitted on the
TXD pin and received on the RXD pin. When a data is received, the stop bit is read in
the RB8 bit in SCON register.
Figure 102. Data Frame Format (Mode 1)
Modes 2 and 3 Modes 2 and 3 are full-duplex, asynchronous modes. The data frame (see Figure 103)
consists of 11 bits: one start bit, eight data bits (transmitted and received LSB first), one
programmable ninth data bit and one stop bit. Serial data is transmitted on the TXD pin
and received on the RXD pin. On receive, the ninth bit is read from RB8 bit in SCON
register. On transmit, the ninth data bit is written to TB8 bit in SCON register. Alterna-
tively, you can use the ninth bit can be used as a command/data flag.
Figure 103. Data Frame Format (Modes 2 and 3)
Transmission (Modes 1, 2
and 3) To initiate a transmission, write to SCON register, set the SM0 and SM1 bits according
to Table 123, and set the ninth bit by writing to TB8 bit. Then, writing the Byte to be
transmitted to SBUF register starts the transmission.
Reception (Modes 1, 2 and 3) To prepare for reception, write to SCON register, set the SM0 and SM1 bits according to
Table 123, and set the REN bit. The actual reception is then initiated by a detected high-
to-low transition on the RXD pin.
TB8
SCON.3
IBRG
CLOCK
RXD
TXD SBUF Tx SR
Rx SR
SM1
SCON.6
SM0
SCON.7
Mode Decoder
M3 M2 M1 M0
RI
SCON.0
TI
SCON.1
Mode & Clock
Controller
SBUF Rx RB8
SCON.2
SM2
SCON.4
T1
CLOCK
PER
CLOCK
Mode 1 D0 D1 D2 D3 D4 D5 D6 D7
Start bit 8-bit data Stop bit
D0 D1 D2 D3 D4 D5 D6 D8
Start bit 9-bit data Stop bit
D7