AVR serial communications
From Just in Time
Notes on getting serial communications to work on AVR.
- 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 fosc/(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.