DOC7707 datasheet(175/307 Pages) ETC1

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175

7707F–AVR–11/10

AT90USB82/162

18.3

SPI Data Modes and Timing

There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which

are determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are

shown in Figure 18-1. Data bits are shifted out and latched in on opposite edges of the XCKn

signal, ensuring sufficient time for data signals to stabilize. The UCPOLn and UCPHAn function-

ality is summarized in Table 18-2. Note that changing the setting of any of these bits will corrupt

all ongoing communication for both the Receiver and Transmitter.

Figure 18-1. UCPHAn and UCPOLn data transfer timing diagrams.

18.4

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.

Table 18-2.

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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