
W77E058A
Publication Release Date: April 17, 2007
- 67 -
Revision A10
After shifting in 9 data bits, there is one more shift to do, after which the SBUF and RB8 are loaded
and RI is set. However certain conditions must be met before the loading and setting of RI can be
done.
1. RI must be 0 and
2. Either SM2 = 0, or the received stop bit = 1.
If these conditions are met, then the stop bit goes to RB8, the 8 data bits go into SBUF and RI is set.
Otherwise the received frame may be lost. After the middle of the stop bit, the receiver goes back to
looking for a 1-to-0 transition on the RxD pin.
14.4 Mode 3
This mode is similar to Mode 2 in all respects, except that the baud rate is programmable. The user
must first initialize the Serial related SFR SCON before any communication can take place. This
involves selection of the Mode and baud rate. The Timer 1 should also be initialized if modes 1 and 3
are used. In all four modes, transmission is started by any instruction that uses SBUF as a destination
register. Reception is initiated in Mode 0 by the condition RI = 0 and REN = 1. This will generate a
clock on the TxD pin and shift in 8 bits on the RxD pin. Reception is initiated in the other modes by the
incoming start bit if REN = 1. The external device will start the communication by transmitting the start
bit.
SBUF
RB8
Transmit Shift Register
Receive Shift
Register
TX SHIFT
PAROUT
D8
Internal
Data Bus
Internal
Data
Bus
TXD
RXD
Serial Port
Interrupt
SMOD=
(SMOD_1)
TCLK
RCLK
SAMPLE
Write to
SBUF
TX START
SOUT
0
1
0
1
TI
1
0
PARIN
STOP
START
LOAD
CLOCK
Read
SBUF
Timer 2 Overflow
(for Serial Port 0 only)
Timer 1
Overflow
TX CLOCK
RI
SERIAL
CONTROLLER
RX CLOCK
LOAD
SBUF
RX
START
RX SHIFT
2
16
1-TO-0
DETECTOR
BIT
DETECTOR
16
CLOCK
SIN
D8
TB8
Figure 23. Serial Port Mode 3