W79E548 Datasheet, PDF (56/79 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E548/W79L548
The 16 states of the counter effectively divide the bit time into 16 slices. The bit detection is done on a
best of three basis. 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. If the first bit detected after the falling edge of RxD pin is not 0, then
this indicates an invalid start bit, and the reception is immediately aborted. The serial port again looks
for a falling edge in the RxD line. If a valid start bit is detected, then the rest of the bits are also
detected and shifted into the SBUF.
After shifting in 8 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.
Timer 1
Overflow
Timer 2 Overflow
(for Serial Port 0 only)
Internal
2
Write to
Data Bus
SBUF
SMOD=
(SMOD_1) ⁰
1
TCLK _{0 1}
16
TX START TX SHIFT
TX CLOCK
TI
RCLK
01
16
SAMPLE
1-TO-0
DETECTOR
SERIAL
CONTROLLER ^RI
RX CLOCK
RX
START
LOAD
SBUF
RX SHIFT
Transmit Shift Register
STOP
PARIN
START ^SOUT
LOAD
CLOCK
RXD
BIT
DETECTOR
CLOCK
PAROUT
SBUF
SIN
D8
RB8
Receive Shift Register
TXD
Serial Port
Interrupt
Read
SBUF
Internal
Data
Bus
Figure 21. Serial Port Mode 1
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