W79E225A Datasheet, PDF (141/200 Pages) Winbond – 8-bit Microcontroller

Preliminary W79E225A/227A Data Sheet
16. SERIAL PORT
The W79E225/227 has two enhanced serial ports that are functionally similar to the serial port of the
original 8052 family. Both the serial ports are full-duplex ports, and the W79E225/227 provides
additional features, such as Frame Error Detection and Automatic Address Recognition. The serial
ports are capable of synchronous and asynchronous communication. In synchronous mode, the
W79E225/227 generates the clock and operates in half-duplex mode. In asynchronous mode, the
serial ports can simultaneously transmit and receive data. The transmit registers and the receive
buffers are both addressed as SBUF (SBUF1 for the second serial port), but any write to
SBUF/SBUF1 writes to the transmit register while any read from SBUF/SBUF1 reads from the receive
buffer. Both serial ports can operate in four modes, as described below. The descriptions are for serial
port 0, however, it also apply to the second serial port.
16.1 Mode 0
This mode provides half-duplex, synchronous communication with external devices. In this mode,
serial data is transmitted and received on the RXD line, and the W79E225/227 provides the shift clock
on TxD, whether the device is transmitting or receiving. Eight bits are transmitted or received per
frame, LSB first. The baud rate is 1/12 or 1/4 of the oscillator frequency, as determined by the SM2 bit
(SCON.5; 0 = 1/12; 1 = 1/4). This programmable baud rate is the only difference between the standard
8051/52 and the W79E225/227 in mode 0.
Any write to SBUF starts transmission. The shift clock is activated, and data is shifted out on RxD until
all eight bits are transmitted. If SM2 is 1, the data appears on RxD one clock period before the falling
edge of the shift clock on TxD. Then, the clock remains low for two clock periods before going high
again. If SM2 is 0, the data appears on RxD three clock periods before the falling edge of the shift
clock on TxD, and the clock on TxD remains low for six clock periods before going high again. This
ensures that, at the receiving end, the data on the RxD line can be clocked on the rising edge of the
shift clock or latched when the clock is low. The TI flag is set high in C1 following the end of
transmission. The functional block diagram is shown below.
Fosc
1/12 1/4
SM2 ₀
1
RI
REN
RXD
P3.0 Alternate
Input Function
Write to
SBUF
TX START
Internal
Data Bus
TX SHIFT
Transmit Shift Register
PARIN
LOAD SOUT
CLOCK
RXD
P3.0 Alternate
Output Function
TX CLOCK
TI
Serial Interrupt
RX CLOCK
RI
SHIFT CLOCK
TX START
LOAD SBUF
RX SHIFT
Serial Controllor
CLOCK
PAROUT
SIN
TXD
P3.1 Alternate
Output Function
Read SBUF
SBUF
Internal
Data Bus
Receive Shift Register
Figure 16-1 Serial Port Mode 0
- 141 -
Publication Release Date: December 14, 2007
Revision A2.0