
121
ATtiny40 [DATASHEET]
8263B–AVR–01/2013
Bits 1:0 – SPR[1:0]: 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 I/O Clock frequency f
clk_I/O 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.
Bits 5:1 – Res: Reserved Bits
These bits are reserved 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 config-
ured as Slave, the SPI is only guaranteed to work at f
clk_I/O/4 or lower.
Table 16-4.
CPHA Functionality
CPHA
Leading Edge
Trailing Edge
0
Sample
Setup
1
Setup
Sample
Table 16-5.
Relationship Between SCK and the I/O Clock Frequency
SPI2X
SPR1
SPR0
SCK Frequency
00
0
f
clk_I/O/4
00
1
f
clk_I/O/16
01
0
f
clk_I/O/64
01
1
f
clk_I/O/128
10
0
f
clk_I/O/2
10
1
f
clk_I/O/8
11
0
f
clk_I/O/32
11
1
f
clk_I/O/64
Bit
7
65
43
21
0
SPIF
WCOL
–
SPI2X
SPSR
Read/Write
R/W
R
R/W
Initial Value
0