AT83SND1CXXX-ROTIL datasheet(135/212 Pages) ATMEL

135 / 212 page

135

4109J–8051–10/06

AT8xC51SND1C

Figure 17-3. Serial I/O Port Block Diagram (Mode 0)

17.3.1

Transmission (Mode 0)

To start a transmission mode 0, write to SCON register clearing bits SM0, SM1.

As shown in Figure 17-4, writing the Byte to transmit to SBUF register starts the transmission.

Hardware shifts the LSB (D0) onto the RXD pin during the first clock cycle composed of a high

level then low level signal on TXD. During the eighth clock cycle the MSB (D7) is on the RXD

pin. Then, hardware drives the RXD pin high and asserts TI to indicate the end of the

transmission.

Figure 17-4. Transmission Waveforms (Mode 0)

17.3.2

Reception (Mode 0)

To start a reception in mode 0, write to SCON register clearing SM0, SM1 and RI bits and setting

the REN bit.

As shown in Figure 17-5, Clock is pulsed and the LSB (D0) is sampled on the RXD pin. The D0

bit is then shifted into the shift register. After eight samplings, the MSB (D7) is shifted into the

shift register, and hardware asserts RI bit to indicate a completed reception. Software can then

read the received Byte from SBUF register.

Figure 17-5. Reception Waveforms (Mode 0)

BRG

CLOCK

TXD

RXD

SBUF Tx SR

SBUF Rx SR

SM1

SCON.6

SM0

SCON.7

Mode Decoder

M3 M2 M1 M0

Mode

Controller

SCON.0

SCON.1

PER

CLOCK

Baud Rate

Controller

Write to SBUF

TXD

RXD

Write to SCON

TXD

RXD

Set REN, Clear RI

AT83SND1CXXX-ROTIL Datasheet(PDF) 135 Page - ATMEL Corporation

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