DOC7707 datasheet(176/307 Pages) ETC1

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7707F–AVR–11/10

AT90USB82/162

18.4.1

USART MSPIM Initialization

The USART in MSPIM mode has to be initialized before any communication can take place. The

initialization process normally consists of setting the baud rate, setting master mode of operation

(by setting DDR_XCKn to one), setting frame format and enabling the Transmitter and the

Receiver. Only the transmitter can operate independently. For interrupt driven USART opera-

tion, the Global Interrupt Flag should be cleared (and thus interrupts globally disabled) when

doing the initialization.

Note:

To ensure immediate initialization of the XCKn output the baud-rate register (UBRRn) must be

zero at the time the transmitter is enabled. Contrary to the normal mode USART operation the

UBRRn must then be written to the desired value after the transmitter is enabled, but before the

first transmission is started. Setting UBRRn to zero before enabling the transmitter is not neces-

sary if the initialization is done immediately after a reset since UBRRn is reset to zero.

Before doing a re-initialization with changed baud rate, data mode, or frame format, be sure that

there is 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 RXCn 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 polling (no interrupts enabled). 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.

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