ATmega16 datasheet(143/315 Pages) ATMEL

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143

ATmega16(L)

2466E–AVR–10/02

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 dis-

abled) 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

and UCSRC.

Note:

1. The example code assumes that the part specific header file is included.

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.

Assembly Code Example

(1)

USART_Init:

; Set baud rate

out

UBRRH, r17

out

UBRRL, r16

; Enable receiver and transmitter

ldi

r16, (1<<RXEN)|(1<<TXEN)

out

UCSRB,r16

; Set frame format: 8data, 2stop bit

ldi

r16, (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)

out

UCSRC,r16

ret

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);

}

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