(cont’d)
Four possible timing relationships may be chosen
by software, using the CPOL and CPHA bits (See
Figure 59).
The idle state of SCK must correspond to
the polarity selected in the SPICSR register (by
pulling up SCK if CPOL = 1 or pulling down SCK if
CPOL = 0).
The combination of the CPOL clock polarity and
CPHA (clock phase) bits selects the data capture
clock edge.
Figure 59 shows an SPI transfer with the four com-
binations of the CPHA and CPOL bits. The dia-
gram may be interpreted as a master or slave tim-
ing diagram where the SCK pin, the MISO pin and
the MOSI pin are directly connected between the
master and the slave device.
: If CPOL is changed at the communication
byte boundaries, the SPI must be disabled by re-
setting the SPE bit.
SCK
(CPOL = 1)
SCK
(CPOL = 0)
MISO
(from master)
MSBit Bit 6 Bit 5 Bit 4 Bit3 Bit 2 Bit 1 LSBit
MOSI
(from slave)
MSBit Bit 6 Bit 5 Bit 4 Bit3 Bit 2 Bit 1 LSBit
SS
(to slave)
CAPTURE STROBE
SCK
(CPOL = 1)
SCK
(CPOL = 0)
MISO
(from master)
MSBit Bit 6 Bit 5 Bit 4 Bit3 Bit 2 Bit 1 LSBit
MOSI
(from slave)
MSBit Bit 6 Bit 5 Bit 4 Bit3 Bit 2 Bit 1 LSBit
SS
(to slave)
CAPTURE STROBE
Note: This figure should not be used as a replacement for parametric information.
Refer to the Electrical Characteristics chapter.
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