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SH79F3283
92
Transmission begins with a “write to SBUF” signal, and it actually commences at the next system clock following the next
rollover in the divide-by-16 counter (divide baud-rate by 16), thus, the bit times are synchronized to the divide-by-16 counter,
not to the “write to SUBF” signal. The start bit is firstly put out on TXD pin, then the 8 bits of data is the next. After all 8 bits of
data in the transmit shift register are transmitted, the stop bit is put out on the TXD pin, and the TI flag is set at the same time
that the stop is send.
Write to SBUF
Shift CLK
D0
D1
D2
D3
D4
D5
D6
D7
TxD
Stop
Start
TI
Send Timing of Mode 1
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. For this purpose RXD is sampled at the rate of 16 times baud rate. When a falling edge is detected, the
divide-by-16 counter is immediately reset. This helps the divide-by-16 counter to synchronize with the serial data of RXD pin.
The divide-by-16 counter divides each bit time into 16 states. The bit detector samples the value of RXD at the 7
th
, 8
th
and 9
th
counter states of each bit time. At least 2 the sampling values have no difference in the state of the three samples, data can be
received This is done for noise rejection. If the first bit after the falling edge of RXD pin is not 0, which indicates an invalid start
bit, and the reception is immediately aborted. The receive circuits are reset and again waiting for a falling edge in the RXD pin.
If a valid start bit is detected, then the rest of the bits are also detected and shifted into the shift register. After shifting in 8 data
bits and the stop bit, the SBUF and RB8 are loaded and RI is set, if the following conditions are met:
1. RI must be 0
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.
At the time, the receiver goes back to looking for another falling edge on the RXD pin. And the user should clear RI by
software for further reception.
Receive Timing of Mode 1
D0
D1
D2
D3
D4
D5
D6
D7
RxD
Stop
Start
Bit Sample
Shift CLK
RI