View source for AVR serial communications

Notes on getting serial communications to work on AVR.
==Registers==
;Enabling USART functions: Set RXEN and TXEN in '''UCSRB''' (USART Control and Status Register B) to enable serial reception.
;Setting the baudrate: '''UBRR''' (USART Baud Rate Register) Sets the baud rate. The baud rate is f<sub>osc</sub>/(UBBR+1) '''divided''' by 2 (?), 8 (U2X = 1) or 16 (U2X = 0). U2X is located in '''UCSRA'''. UBRR is divided into two 8-bit registers ('''UBRRH''', '''UBRRL''').
;Choosing the frame format: UCSZ0 - UCSZ1 (in '''UCSRC'''), and  UCSZ2 (in '''UCSRB''') can be used to set the number of databits (USART Character Size: 5, 6, 7, 8, ''reserved'',  ''reserved'', ''reserved'',9) . UPM0 - UPM1 (USART Parity mode: none, ''reserved'', even, odd) and USBS (USART Stop Bit Select: one bit, two bit) are in '''UCSRC'''. Default is ''8n1''. '''Note''': on atmega16, al writes to UCSRC must have bit ''URSEL'' set, otherwise the write will redirect to the UBRRH register.
;Receiving data (synchronously): Can be performed by waiting until RXC in '''UCSRA''' has been set and then reading the '''UDR''' register. Reading from UDR will clear RXC.
;Sending data: can be performed by first checking bit UDRE in '''UCSRA''' and then writing to '''UDR'''.
;Interrupt driven reception: This can be achieved by implementing the ISR for USART (''USART_RXC_vect''), enabling the RXC interrupt by setting RXCIE in '''USCRB''' and allowing interrupts globally, by using the '''''sei()'''''-macro.