80C186EC/80C188EC Microprocessor User's Manual

CHAPTER 11
SERIAL COMMUNICATIONS UNIT
11.1 INTRODUCTION
The Serial Communications Unit is composed of two identical serial ports, or channels. Each serial
port is independent of the other. This chapter describes the operation of a single serial port.
The serial port implements several industry-standard asynchronous communications protocols,
and it readily interfaces to many different processors over a standard serial interface. Several processors
and systems can be connected to a common serial bus using a multiprocessor protocol.
The serial port also implements a simple synchronous protocol. The synchronous protocol is most
commonly used to expand the number of I/O pins with shift registers.
Features:
• Full duplex operation
• Programmable seven, eight or nine data bits in asynchronous mode
• Independent baud rate generator
• Maximum baud rate of 1/16 the processor clock
• Double-buffered transmit and receive
• Clear-to-Send feature for transmission
• Break character transmission and detection
• Programmable even, odd or no parity
• Detects both framing and overrun errors
• Supports interrupt on transmit and receive
11.1.1 Asynchronous Communications
Asynchronous communications protocols allow different devices to communicate without a common
reference clock. The devices communicate at a common baud rate, or bits per second. Data
is transmitted and received in frames. A frame is a sequence of bits shifted serially onto or off the
communications line.
Each asynchronous frame consists of a start bit (always a logic zero), followed by the data bits
and a terminating stop bit. The serial port can transmit and receive seven, eight or nine data bits.
The last data bit can optionally be replaced by an even or odd parity bit. Figure 11-1 shows a typical
10-bit frame.
11-1