AT90USB646 datasheet(209/434 Pages) ATMEL

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7593A–AVR–02/06

AT90USB64/128

ality is summarized in Table 19-2. Note that changing the setting of any of these bits will corrupt

all ongoing communication for both the Receiver and Transmitter.

Figure 19-1. UCPHAn and UCPOLn data transfer timing diagrams.

19.4 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 UDORDn bit in UCSRnC 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 UDRn. A UART transmit com-

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

19.4.1

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-

Table 19-2.

UCPOLn and UCPHAn Functionality-

UCPOLn

UCPHAn

SPI Mode

Leading Edge

Trailing Edge

Sample (Rising)

Setup (Falling)

Setup (Rising)

Sample (Falling)

Setup (Rising)

Setup (Falling)

Sample (Rising)

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

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