Intel 8XC251SB, 8XC251SA, 8XC251SP manual Serial Port Signals, Function Type Description Multiplexed

CHAPTER 10

SERIAL I/O PORT

The serial input/output port supports communication with modems and other external peripheral devices. This chapter provides instructions for programming the serial port and generating the se- rial I/O baud rates with timer 1 and timer 2.

10.1 OVERVIEW

The serial I/O port provides both synchronous and asynchronous communication modes. It oper- ates as a universal asynchronous receiver and transmitter (UART) in three full-duplex modes (modes 1, 2, and 3). Asynchronous transmission and reception can occur simultaneously and at different baud rates. The UART supports framing-bit error detection, multiprocessor communi- cation, and automatic address recognition. The serial port also operates in a single synchronous mode (mode 0).

The synchronous mode (mode 0) operates at a single baud rate. Mode 2 operates at two baud rates. Modes 1 and 3 operate over a wide range of baud rates, which are generated by timer 1 and timer 2. Baud rates are detailed in section 10.6, “Baud Rates.”

The serial port signals are defined in Table 10-1, and the serial port special function registers are described in Table 10-2. Figure 10-1 is a block diagram of the serial port.

For the three asynchronous modes, the UART transmits on the TXD pin and receives on the RXD pin. For the synchronous mode (mode 0), the UART outputs a clock signal on the TXD pin and sends and receives messages on the RXD pin (Figure 10-1). The SBUF register, which holds re- ceived bytes and bytes to be transmitted, actually consists of two physically different registers. To send, software writes a byte to SBUF; to receive, software reads SBUF. The receive shift reg- ister allows reception of a second byte before the first byte has been read from SBUF. However, if software has not read the first byte by the time the second byte is received, the second byte will overwrite the first. The UART sets interrupt bits TI and RI on transmission and reception, respec- tively. These two bits share a single interrupt request and interrupt vector.

Table 10-1. Serial Port Signals

Function	Type	Description	Multiplexed
Name			With
TXD	O	Transmit Data. In mode 0, TXD transmits the clock signal. In	P3.1
		modes 1, 2, and 3, TXD transmits serial data.
RXD	I/O	Receive Data. In mode 0, RXD transmits and receives serial	P3.0
		data. In modes 1, 2, and 3, RXD receives serial data.

10-1