This bit causes the SPI interrupt to be executed if the SPIF bit in the SPSR register is set and if the global interrupt enable bit in SREG is set.
Bit 6 – SPE SPI Enable
When the SPE bit is written to ‘1’, the SPI is enabled. This bit must be set to enable any SPI operations.
Bit 5 – DORD Data Order
If the DORD bit is written to ‘1’, the LSB of the data word is transmitted first. If the DORD bit is written to ‘0’, the MSB of the data word is transmitted first.
Bit 4 – MSTR Host/Client Select
This bit selects SPI Host mode when written to ‘1’ and client SPI mode when written to logic ‘0’. If NSS is configured as an input and is driven low while MSTR is set, MSTR is cleared and SPIF in SPSR is set. The user, then, has to set MSTR to re-enable SPI Host mode.
Bit 3 – CPOL Clock Polarity
If this bit is written to
‘1’, SCK is high when idle. If CPOL is written to
‘0’, SCK is low when idle. For example, see SPI Transfer
Format with CPHA = 0 and SPI Transfer Format with CPHA = 1 in the
Data Modes from Related Links. The CPOL functionality is summarized in
the following table.
Table 4-88. CPOL
Functionality
CPOL
Leading Edge
Trailing Edge
0
Rising
Falling
1
Falling
Rising
Bit 2 – CPHA Clock Phase
The setting of the clock phase (CPHA)
determines if data is sampled on the leading (first) or trailing (last) edge of SCK.
For example, see SPI Transfer Format with CPHA = 0 and SPI Transfer Format
with CPHA = 1 in the Data Modes from Related Links. The CPHA
functionality is summarized in the following table.
Table 4-89. CPHA
Functionality
CPHA
Leading Edge
Trailing Edge
0
Sample
Setup
1
Setup
Sample
Bits 1:0 – SPR[1:0] SPI Clock Rate Select
These two bits control the SCK rate of the device configured as a host. SPR[1:0] has no effect on the client. The relationship between SCK and the CLKI/O is shown in the following table.
Table 4-90. Relationship Between SCK and Oscillator Frequency
SPSR.SPI2X
SPR1
SPR0
SCK Frequency
0
0
0
CLKI/O/4
0
0
1
CLKI/O/16
0
1
0
CLKI/O/64
0
1
1
CLKI/O/128
1
0
0
CLKI/O/2
1
0
1
CLKI/O/8
1
1
0
CLKI/O/32
1
1
1
CLKI/O/64
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