Avr307-AtmelCorporation.PDFVIP

AVR307: Half Duplex UART Using the USI Module Features 8-bit • Half Duplex UART Communication • Communication Speed Up To 230.4 kbps at 14.75MHz Microcontrollers • Interrupt Controlled Communication • Eight Bit Data, One Stop-bit, No Parity • Data Buffer Handling Application Note Introduction The Universal Serial Interface (USI) present in AVR devices like the ATtiny26, ATtiny2313, and ATmega169, is a communication module designed for TWI and SPI communication. The USI is however not restricted to these two serial communication standards. It can be used for UART communication as well. By using the USI for UART communication one can avoid some of the inconveniences associated with software implemented UART communication. By using the USI, inter- rupts control the communication and the bits are automatically shifted out. In this way, less processing time is occupied by the communication, freeing processing time for other parts of the application while communicating. This document describes how to use the USI of the ATtiny26 for UART communica- tion. Source code for communication drivers for transmission and reception is provided. The drivers can be adapted to the ATtiny2313 or ATmega169 USI modules by minor changes to the code. The code is complete with both data buffer handling and combined transmitter and receiver. These features can be split or removed to optimize the use of resources. Rev. 4300A–AVR–10/03 Theory of Operation The UART protocol is an asynchronous serial communication standard. The frame for- mat is specified as a start bit followe