Chapter 8 - v3.1 - Chapter 8 Serial Input/Output...

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88 Chapter 8 Serial Input/Output Asynchronous Serial Communications Serial systems use a single line to transfer data bits one after the other. This reduces the number of lines but slows down the rate of data transfer. The term asynchronous refers to the fact that each device uses its own clock and the clocks do not need to be synchronized. The asynchronous I/O subsystem for the 68HC11 is called the serial communications interface (SCI). The SCI subsystem is used primarily for serial data communications. In asynchronous serial communications, the communicating devices are remote (with a distance greater than a few meters), for example they may communicate over a telephone line. An asynchronous communication system (or network) must have at least one transmitter and one or more receivers. A transmitter sends data to be read by the receivers. Start and Stop Framing In asynchronous serial communications, the basic unit (group of bits) of communication is the character or data frame (normally a byte). The transmitter can send frames at any rate and whenever it feels like. Hence there is a need for the receiver to know when a frame starts and ends. So each character is framed by start and stop bits. In the figure given below a constant high signal is considered to be idle with no data being transmitted; a start bit is always zero (0) and a stop bit is always a one (1); data bits are transmitted with the LSB first; and voltage does not return to zero between adjacent 1 bits.
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89 Parity It is used to detect single bit errors in communication. It refers to the quantity of 1 bits in a binary number. The transmitter generates the parity and the receiver checks it. Parity bit is either a 0 or 1 depending on: 1. the type of parity selected, even or odd 2. the number of 1 bits in the data byte to be transferred Example: (even parity convention) Assume that the transmitter wants to send the seven bit ASCII code 0111000 with even parity. Then the transmitted bits (shown in order of transmission from left to right) are as follows: 0 0001110 1 1 Start Data Parity Stop Bit bits bit bit The number of one bits in the above frame excluding the start and stop bits but including the parity is even. Some possible framing configurations Data Speed and Baud Rate Serial data speed is the number of bits transmitted per second (bps). Baud is the rate at which the signal changes. If each signal element represents one bit, then the data speed and baud rate are the same. In some cases communication requires extra signal bits that are not data. In other cases, a signal element may encode several bits. If each signal element represents more than one bit, then the data speed and the baud rate are not equal to each other. In 6811 SCI, each signal element encodes 1 bit.
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90 Baud rate includes the start, parity and stop bits. In asynchronous serial communications terminology, for example, 9600 baud means that serial data (including the start and stop bits) is transmitted and received at the rate of 9600 bits per second. Since there are overheads in the frame
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This note was uploaded on 01/30/2011 for the course EE 447 taught by Professor Skst during the Fall '10 term at Middle East Technical University.

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Chapter 8 - v3.1 - Chapter 8 Serial Input/Output...

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