User Manual

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
21.5.1 USART MSPIM Initialization
The USART in MSPIM mode has to be initialized before any communication can take place. The initialization pro-
cess 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 indepen-
dently. For interrupt driven USART operation, 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 necessary 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 ongo-
ing 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 function that are equal in
functionality. The examples assume polling (no interrupts enabled). The baud rate is given as a function parame-
ter. For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 registers.