AT90USB646 datasheet(175/456 Pages) ATMEL

175 / 456 page

175

7593L–AVR–09/12

AT90USB64/128

corresponding interrupt handling vector. Alternatively, the SPIF bit is cleared by first reading the

SPI Status Register with SPIF set, then accessing the SPI Data Register (SPDR).

• Bit 6 – WCOL: Write COLlision Flag

The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The

WCOL bit (and the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set,

and then accessing the SPI Data Register.

• Bit 5..1 – Res: Reserved bits

These bits are reserved bits in the Atmel AT90USB64/128 and will always read as zero.

• Bit 0 – SPI2X: Double SPI Speed bit

When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI

is in Master mode (see Table 18-4 on page 174). This means that the minimum SCK period will

be two CPU clock periods. When the SPI is configured as Slave, the SPI is only guaranteed to

work at f

osc/4 or lower.

The SPI interface on the AT90USB64/128 is also used for program memory and EEPROM

downloading or uploading. See page 373 for serial programming and verification.

18.1.5

SPDR – SPI Data Register

The SPI Data Register is a read/write register used for data transfer between the Register File

and the SPI Shift Register. Writing to the register initiates data transmission. Reading the regis-

ter causes the Shift Register Receive buffer to be read.

18.2

Data modes

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

determined by control bits CPHA and CPOL. The SPI data transfer formats are shown in Figure

18-3 on page 176 and Figure 18-4 on page 176. Data bits are shifted out and latched in on

opposite edges of the SCK signal, ensuring sufficient time for data signals to stabilize. This is

clearly seen by summarizing Table 18-2 on page 174 and Table 18-3 on page 174, as done

below:

Bit

MSB

LSB

SPDR

Read/write

R/W

Initial value

Undefined

Table 18-5.

CPOL functionality.

Leading edge

Trailing edge

SPI mode

CPOL=0, CPHA=0

Sample (rising)

Setup (falling)

CPOL=0, CPHA=1

Setup (rising)

Sample (falling)

CPOL=1, CPHA=0

Sample (falling)

Setup (rising)

CPOL=1, CPHA=1

Setup (falling)

Sample (rising)

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