Rainbow Electronics T89C5121 - Manual (Page 89)

89
A/T8xC5121
4164G–SCR–07/06
Asynchronous Modes
(Modes 1, 2 and 3) The Serial Port has one 8-bit and two 9-bit asynchronous modes of operation. Figure 43
shows the Serial Port block diagram in such asynchronous modes.
Figure 43. 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 44) 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 44. Data Frame Format (Mode 1)
Modes 2 and 3 Modes 2 and 3 are full-duplex, asynchronous modes. The data frame (see Figure 45)
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 as a command/data flag.
Figure 45. Data Frame Format (Modes 2 and 3)
Transmission (Modes 1, 2
and 3) To initiate a transmission, write to SCON register, setting SM0 and SM1 bits according
to Table 64, and setting 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 a reception, write to SCON register, setting SM0 and SM1 bits according
to Table 64, and setting 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
Modes 2 and 3 D0 D1 D2 D3 D4 D5 D6 D8
Start Bit 9-bit Data Stop Bit
D7