USART Initialization
The USART has to be initialized before any communication can take place. The initial-
ization process normally consists of setting the baud rate, setting frame format and
enabling the Transmitter or the Receiver depending on the usage. For interrupt driven
USART operation, the Global Interrupt Flag should be cleared (and interrupts globally
disabled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that
there are no ongoing transmissions during the period the registers are changed. The
TXC Flag can be used to check that the Transmitter has completed all transfers, and the
RXC Flag can be used to check that there are no unread data in the receive buffer. Note
that the TXC Flag must be cleared before each transmission (before UDR is written) if it
is used for this purpose.
The following simple USART initialization code examples show one assembly and one
C function that are equal in functionality. The examples assume asynchronous opera-
tion using polling (no interrupts enabled) and a fixed frame format. The baud rate is
given as a function parameter. For the assembly code, the baud rate parameter is
assumed to be stored in the r17:r16 registers. When the function writes to the UCSRC
Register, the URSEL bit (MSB) must be set due to the sharing of I/O location by UBRRH
and UCSRC.
Assembly Code Example(1)
USART_Init:
; Set baud rate
out
out
UBRRH, r17
UBRRL, r16
; Enable receiver and transmitter
ldi
out
r16, (1<<RXEN)|(1<<TXEN)
UCSRB,r16
; Set frame format: 8data, 2stop bit
ldi
out
ret
r16, (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)
UCSRC,r16
C Code Example(1)
void USART_Init( unsigned int baud )
{
/* Set baud rate */
UBRRH = (unsigned char)(baud>>8);
UBRRL = (unsigned char)baud;
/* Enable receiver and transmitter */
UCSRB = (1<<RXEN)|(1<<TXEN);
/* Set frame format: 8data, 2stop bit */
UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0);
}
Note:
1. See “About Code Examples” on page 7.
More advanced initialization routines can be made that include frame format as parame-
ters, disable interrupts and so on. However, many applications use a fixed setting of the
Baud and Control Registers, and for these types of applications the initialization code
can be placed directly in the main routine, or be combined with initialization code for
other I/O modules.
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ATMEL [ ATMEL ]
