139
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
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 clkIO frequency fclkio is shown in the following table:
15.2.5 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 ATmega16/32/64/M1/C1 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 15-4). 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 fclkio/4 or lower.
The SPI interface on the ATmega16/32/64/M1/C1 is also used for program memory and EEPROM downloading or uploading.
15.2.6 SPI Data Register – SPDR
Bits 7:0 - SPD7:0: SPI Data
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 register causes the shift register receive buffer to be read.
Table 15-4. Relationship Between SCK and the Oscillator Frequency
SPI2X
SPR1
SPR0
SCK Frequency
0
fclkio/4
0
1
f
clkio/16
0
1
0
f
clkio/64
0
1
fclkio/128
1
0
f
clkio/2
1
0
1
f
clkio/8
1
0
fclkio/32
1
f
clkio/64
Bit
765
432
10
SPIF
WCOL
–
SPI2X
SPSR
Read/Write
R
R/W
Initial Value
000
00
Bit
7654321
0
SPD7
SPD6
SPD5
SPD4
SPD3
SPD2
SPD1
SPD0
SPDR
Read/Write
R/W
Initial Value
XXX
XXXXX
Undefined