Data Sheet
168
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
19.5 Register Description
19.5.1 SPCR – SPI Control Register
• Bit 7 – SPIE: SPI Interrupt Enable
This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and the if the Global
Interrupt Enable bit in SREG is set.
• Bit 6 – SPE: SPI Enable
When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI operations.
• Bit 5 – DORD: Data Order
When the DORD bit is written to one, the LSB of the data word is transmitted first.
When the DORD bit is written to zero, the MSB of the data word is transmitted first.
• Bit 4 – MSTR: Master/Slave Select
This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic zero. If SS
is
configured as an input and is driven low while MSTR is set, MSTR will be cleared, and SPIF in SPSR will
become set. The user will then have to set MSTR to re-enable SPI Master mode.
• Bit 3 – CPOL: Clock Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low when idle.
Refer to Figure 19-3 and Figure 19-4 for an example. The CPOL functionality is summarized below:
• Bit 2 – CPHA: Clock Phase
The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or trailing (last)
edge of SCK. Refer to Figure 19-3 and Figure 19-4 for an example. The CPOL functionality is summarized
below:
• 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:
Bit 76543210
0x2C (0x4C) SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 SPCR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 19-3. CPOL Functionality
CPOL Leading Edge Trailing Edge
0 Rising Falling
1 Falling Rising
Table 19-4. CPHA Functionality
CPHA Leading Edge Trailing Edge
0 Sample Setup
1 Setup Sample