W79E825A Datasheet, PDF (83/127 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E825A/824A/823B/822B Data Sheet
16.3 MODE 2
This mode uses a total of 11 bits in asynchronous full-duplex communication. The functional
description is shown in the figure below. The frame consists of one start bit (0), 8 data bits (LSB first),
a programmable 9th bit (TB8) and a stop bit (0). The 9th bit received is put into RB8. The baud rate is
programmable to 1/32 or 1/64 of the oscillator frequency, which is determined by the SMOD bit in
PCON SFR. Transmission begins with a write to SBUF. The serial data is brought out on to TxD pin at
C1 following the first roll-over of the divide-by-16 counter. The next bit is placed on TxD pin at C1
following the next rollover of the divide-by-16 counter. Thus the transmission is synchronized to the
divide-by-16 counter, and not directly to the write to SBUF signal. After all 9 bits of data are
transmitted, the stop bit is transmitted. The TI flag is set in the C1 state after the stop bit has been put
out on TxD pin. This will be at the 11th rollover of the divide-by-16 counter after a write to SBUF.
Reception is enabled only if REN is high. The serial port actually starts the receiving of serial data,
with the detection of a falling edge on the RxD pin. The 1-to-0 detector continuously monitors the RxD
line, sampling it at the rate of 16 times the selected baud rate. When a falling edge is detected, the
divide- by-16 counter is immediately reset. This helps to align the bit boundaries with the rollovers of
the divide-by-16 counter. The 16 states of the counter effectively divide the bit time into 16 slices. The
bit detection is done on a best of three bases. The bit detector samples the RxD pin, at the 8th, 9th
and 10th counter states. By using a majority 2 of 3 voting system, the bit value is selected. This is
done to improve the noise rejection feature of the serial port.
Figure 16-3: Serial Port Mode 2
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Publication Release Date: February 21, 2008
Revision A9