
Chapter 14. Timers/Counters and Serial Interface Page 14-15
14.3.3 Mode 0
Serial mode 0 provides synchronous, half-duplex serial communication. For Serial Port 0, serial
data output occurs on the RXD0OUT pin, serial data is received on the RXD0 pin, and the TXD0
pin provides the shift clock for both transmit and receive. For Serial Port 1, the corresponding pins
are RXD1OUT, RXD1, and TXD1.
The serial mode 0 baud rate is either CLKOUT/12 or CLKOUT/4, depending on the state of the
SM2_0 bit (or SM2_1 for Serial Port 1). When SM2_0 = 0, the baud rate is CLKOUT/12, when
SM2_0 = 1, the baud rate is CLKOUT/4.
Mode 0 operation is identical to the standard 8051. Data transmission begins when an instruction
writes to the SBUF0 (or SBUF1) SFR. The USART shifts the data, LSB first, at the selected baud
rate, until the 8-bit value has been shifted out.
Mode 0 data reception begins when the REN_0 (or REN_1) bit is set and the RI_0 (or RI_1) bit is
cleared in the corresponding SCON SFR. The shift clock is activated and the USART shifts data,
LSB first, in on each rising edge of the shift clock until 8 bits have been received. One CLKOUT
cycle after the 8th bit is shifted in, the RI_0 (or RI_1) bit is set and reception stops until the software
clears the RI bit.
Figure 14-7 through Figure 14-10 illustrate Serial Port Mode 0 transmit and receive timing for both
low-speed (CLKOUT/12) and high-speed (CLKOUT/4) operation. The figures show Port 0 signal
names, RXD0, RXD0OUT, and TXD0. The timing is the same for Port 1 signals RXD1, RXD1OUT,
and TXD1, respectively.
Commentaires sur ces manuels