AT90SCR100 datasheet(259/433 Pages) ATMEL

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TPR0327AY–SMS–30Jan09

AT90SCR100

Figure 19-1.

UCPHA0 and UCPOL0 data transfer timing diagrams.

19.5

Frame Formats

A serial frame for the MSPIM is defined to be one character of 8 data bits. The USART in MSPIM

mode has two valid frame formats:

• 8-bit data with MSB first

• 8-bit data with LSB first

A frame starts with the least or most significant data bit. Then the next data bits, up to a total of

eight, are succeeding, ending with the most or least significant bit accordingly. When a complete

frame is transmitted, a new frame can directly follow it, or the communication line can be set to

an idle (high) state.

The UDORD0 bit in UCSR0C sets the frame format used by the USART in MSPIM mode. The

Receiver and Transmitter use the same setting. Note that changing the setting of any of these

bits will corrupt all ongoing communication for both the Receiver and Transmitter.

16-bit data transfer can be achieved by writing two data bytes to UDR0. A UART transmit com-

plete interrupt will then signal that the 16-bit value has been shifted out.

19.5.1

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_XCK0 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.

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 TXC0 Flag

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

UCPOL=0

UCPOL=1

To ensure immediate initialization of the XCK0 output the baud-rate register

(UBRR0) must be zero at the time the transmitter is enabled. Contrary to the nor-

mal mode USART operation the UBRR0 must then be written to the desired value

after the transmitter is enabled, but before the first transmission is started. Setting

UBRR0 to zero before enabling the transmitter is not necessary if the initialization

is done immediately after a reset since UBRR0 is reset to zero.

Note

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