Serial Communications in AVR

Serial USART

The serial USART is used for full duplex (two-way) communication between a receiver and transmitter. This is accomplished by equipping the ATmega16 with independent hardware for the transmitter and receiver. The USART is typically used for asynchronous communication. That is, there is not a common clock between the transmitter and receiver to keep them synchronized with one another. To maintain synchronization between the transmitter and receiver, framing start and stop bits are used at the beginning and end of each data byte in a transmission sequence. The ATmega16 USART is quite flexible. It has the capability to be set to a variety of data transmission rates known as the baud (bits per second) rate. The USART may also be set for data bit widths of 5 to 9 bits with one or two stop bits. Furthermore, the ATmega16 is equipped with a hardware-generated parity bit (even or  odd) and parity check hardware at the receiver. A single parity bit allows for the detection of a single bit error within a byte of data. The USART may also be configured to operate in a synchronous mode.

Serial Peripheral Interface

The ATmega16 SPI can also be used for two-way serial communication between a transmitter and a receiver. In the SPI system, the transmitter and receiver share a common clock source. This requires an additional clock line between the transmitter and receiver but allows for higher data transmission rates as compared with the USART. The SPI may be viewed as a synchronous 16-bit shift register with an 8-bit half residing in the transmitter and the other 8-bit half residing in the receiver. The transmitter is designated the master because it provides the synchronizing clock source between the transmitter and the receiver. The receiver is designated as the slave.

Two-Wire Serial Interface

The TWI subsystem allows the system designer to network a number of related devices (microcontrollers, transducers, displays, memory storage, etc.) together into a system using a two-wire interconnecting scheme. The TWI allows a maximum of 128 devices to be connected together. Each device has its own unique address and may both transmit and receive over the two-wire bus at frequencies up to 400 kHz. This allows the device to freely exchange information with other devices in the network within a small area.

Analog-to-Digital Converter

The ATmega16 is equipped with an eight-channel ADC subsystem. The ADC converts an analog signal from the outside world into a binary representation suitable for use by the microcontroller. The ATmega16 ADC has 10-bit resolution. This means that an analog voltage between 0 and 5 V will be encoded into one of 1024 binary representations between (000)16 and (3FF)16. This provides the ATmega16 with a voltage resolution of approximately 4.88 mV.