ATmega88 datasheet(163/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

16.1.4

SPI Status Register – SPSR

• Bit 7 – SPIF: SPI Interrupt Flag

When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in

SPCR is set and global interrupts are enabled. If SS is an input and is driven low when the SPI is

in Master mode, this will also set the SPIF Flag. SPIF is cleared by hardware when executing the

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 ATmega48/88/168 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 16-4). 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 ATmega48/88/168 is also used for program memory and EEPROM

downloading or uploading. See page 293 for serial programming and verification.

16.1.5

SPI Data Register – SPDR

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.

16.2

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

16-3 and Figure 16-4. Data bits are shifted out and latched in on opposite edges of the SCK sig-

nal, ensuring sufficient time for data signals to stabilize. This is clearly seen by summarizing

Table 16-2 and Table 16-3, as done below.

Bit

76543210

SPIF

WCOL

–––––

SPI2X

SPSR

Read/Write

RRRRRRR

R/W

Initial Value

00000000

Bit

7654

3210

MSB

LSB

SPDR

Read/Write

R/W

Initial Value

XXXX

Undefined

ATmega88 Datasheet(PDF) 163 Page - ATMEL Corporation

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