
Preliminary W77E468
Publication Release Date: January 1999
- 65 - Revision A1
SBUF
Transmit Shift Register
Receive Shift Register
TX
SHIFT
PAROUT
Internal
Data Bus
Internal
Data Bus
RXD
P3.0 Alternate
Output Function
TXD
P3.1 Alternate
Output function
RXD
P3.0 Alternate
Iutput function
Serial Port Interrupt
Write to
SBUF
TX
START
SOUT
TI
RI
REN
PARIN
LOAD
CLOCK
Read SBUF
TX CLOCK
RI
SERIAL
CONTROLLE
SHIFT
CLOCK
RX
CLOCK
LOAD SBUF
RX
START
RX SHIFT
CLOCK
SIN
4
12
1
0
SM2
Clock Source
Mode input
div. by 4 osc/1
div. by 64 osc/16
div. by 1024 osc/256
Figure 20. Serial Port Mode 0
The TI flag is set high in C1 following the end of transmission of the last bit. The serial port will
receive data when REN is 1 and RI is zero. The shift clock (TxD) will be activated and the serial port
will latch data on the rising edge of shift clock. The external device should therefore present data on
the falling edge on the shift clock. This process continues till all the 8 bits have been received. The RI
flag is set in C1 following the last rising edge of the shift clock on TxD. This will stop reception, till the
RI is cleared by software.
MODE 1
In Mode 1, the full duplex asynchronous mode is used. Serial communication frames are made up of
10 bits transmitted on TXD and received on RXD. The 10 bits consist of a start bit (0), 8 data bits
(LSB first), and a stop bit (1). On receive, the stop bit goes into RB8 in the SFR SCON. The baud rate
in this mode is variable. The serial baud can be programmed to be 1/16 or 1/32 of the Timer 1
overflow. Since the Timer 1 can be set to different reload values, a wide variation in baud rates is
possible.