Serial Communication Terminology

Asynchronous versus Synchronous Serial Transmission

In serial communications, the transmitting and receiving device must be synchronized to one another and use a common data rate and protocol. Synchronization allows both the transmitter and receiver to be expecting data transmission/reception at the same time. There are two basic methods of maintaining ‘‘sync’’ between the transmitter and receiver: asynchronous and synchronous. In an asynchronous serial communication system, such as the USART aboard theATmega16, framing bits are used at the beginning and end of a data byte. These framing bits alert the receiver that an incoming data byte has arrived and also signals the completion of the data byte reception. The data rate for an asynchronous serial system is typically much slower than the synchronous system, but it only requires a single wire between the transmitter and receiver. A synchronous serial communication system maintains ‘‘sync’’ between the transmitter and receiver by employing a common clock between the two devices.Data bits are sent and received on the edge of the clock. This allows data transfer rates higher than with asynchronous techniques but requires two lines, data and clock, to connect the receiver and transmitter.

Baud Rate

Data transmission rates are typically specified as a baud or bits per second rate. For example, 9600 baud indicates data are being transferred at 9600 bits per second.

Full Duplex

Often, serial communication systemsmust both transmit and receive data. To do both transmission and reception simultaneously requires separate hardware for transmission and reception. A single duplex system has a single complement of hardware that must be switched from transmission to reception configuration. A full duplex serial communication system has separate hardware for transmission and reception.

Nonreturn to Zero Coding Format

There aremany different coding standards used within serial communications. The important point is the transmitter and receiver must use a common coding standard so data may be interpreted correctly at the receiving end. The Atmel ATmega16 uses a nonreturn to zero coding standard. In nonreturn to zero, coding a logic 1 is signaled by a logic high during the entire time slot allocated for a single bit, whereas a logic 0 is signaled by a logic low during the entire time slot allocated for a single bit.

The RS-232Communication Protocol

When serial transmission occurs over a long distance, additional techniques may be used to ensure data integrity. Over long distances, logic levels degrade and may be corrupted by noise. At the receiving end, it is difficult to discern a logic high from a logic low. The RS-232 standard has been around for some time. With the RS-232 standard (EIA-232), a logic 1 is represented with a −12-VDC level, whereas a logic 0 is represented by a +12-VDC level. Chips are commonly available (e.g., MAX232) that convert the 5- and 0-V output levels from a transmitter to RS-232- compatible levels and convert back to 5- and 0-V levels at the receiver. The RS-232 standard also specifies other features for this communication protocol.

Parity

To further enhance data integrity during transmission, parity techniques may be used. Parity is an additional bit (or bits) that may be transmitted with the data byte. The ATmega16 uses a single parity bit. With a single parity bit, a single-bit error may be detected. Parity may be even or odd. In even parity, the parity bit is set to 1 or 0, such that the number of 1’s in the data byte including the parity bit is even. Meaning number of 1’s in 8 data bit + that in parity bit must be even. In odd parity, the parity bit is set to 1 or 0, such that the number of 1’s in the data byte including the parity bit is odd. Meaning number of 1’s in 8 data bits + that in parity bit must be odd. At the receiver, the number of bits within a data byte including the parity bit are counted to ensure that parity has not changed, indicating an error, during transmission.