ATmega1284P datasheet(169/380 Pages) ATMEL

169 / 380 page

169

8059D–AVR–11/09

ATmega1284P

• Bits 1:0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0

These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have

no effect on the Slave. The relationship between SCK and the Oscillator Clock frequency f

osc is

shown in the following table:

16.5.2

SPSR – SPI Status Register

• 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 ATmega1284P 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 16-5). 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 ATmega1284P is also used for program memory and EEPROM down-

loading or uploading. See page 306 for serial programming and verification.

Table 16-5.

Relationship Between SCK and the Oscillator Frequency

SPI2X

SPR1

SPR0

SCK Frequency

osc/4

osc/16

osc/64

osc/128

osc/2

osc/8

osc/32

osc/64

Bit

76543210

0x2D (0x4D)

SPIF

WCOL

–––––

SPI2X

SPSR

Read/Write

RRRRRRR

R/W

Initial Value

00000000

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