DOC7707 datasheet(152/307 Pages) ETC1

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7707F–AVR–11/10

AT90USB82/162

17.4

USART Initialization

The USART has to be initialized before any communication can take place. The initialization pro-

cess 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 TXCn 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 TXCn Flag must be

cleared before each transmission (before UDRn is written) if it is used for this purpose.

The following simple USART initialization code examples show one assembly and one C func-

tion that are equal in functionality. The examples assume asynchronous operation 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.

Note:

1. See “About Code Examples” on page 7.

More advanced initialization routines can be made that include frame format as parameters, dis-

able 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

UBRRHn, r17

out

UBRRLn, r16

; Enable receiver and transmitter

ldi

r16, (1<<RXENn)|(1<<TXENn)

out

UCSRnB,r16

; Set frame format: 8data, 2stop bit

ldi

r16, (1<<USBSn)|(3<<UCSZn0)

out

UCSRnC,r16

ret

C Code Example

(1)

void

USART_Init( unsigned int baud )

{

/* Set baud rate */

UBRRHn = (unsigned char)(baud>>8);

UBRRLn = (unsigned char)baud;

/* Enable receiver and transmitter */

UCSRnB = (1<<RXENn)|(1<<TXENn);

/* Set frame format: 8data, 2stop bit */

UCSRnC = (1<<USBSn)|(3<<UCSZn0);

}

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