126
8008H–AVR–04/11
ATtiny48/88
Data bits are shifted out and latched in on opposite edges of the SCK signal, ensuring sufficient
time for data signals to stabilize. This is clearly seen by summarizing Table 14-3 on page 126
14.5
Register Description
14.5.1
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 Mas-
ter 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 14-3 and Figure 14-4 for an example. The CPOL functionality is sum-
marized below:
Table 14-2.
Setting SPI Mode using Control Bits CPOL and CPHA
CPOL
CPHA
SPI Mode
Leading Edge
Trailing eDge
0
Sample (Rising)
Setup (Falling)
0
1
Setup (Rising)
Sample (Falling)
1
0
2
Sample (Falling)
Setup (Rising)
1
3
Setup (Falling)
Sample (Rising)
Bit
7
654
32
10
SPIE
SPE
DORD
MSTR
CPOL
CPHA
SPR1
SPR0
SPCR
Read/Write
R/W
Initial Value
0
Table 14-3.
CPOL Functionality
CPOL
Leading Edge
Trailing Edge
0
Rising
Falling
1
Falling
Rising
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