Data Modes
There are four combinations of SCK phase and polarity with respect to serial data, which are
determined by control bits CPHA and CPOL. The SPI data transfer formats are shown in Figure
77 and Figure 78. 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
70 and Table 71, as done below:
Table 73. CPOL and CPHA Functionality
Leading edge
Sample (Rising)
Setup (Rising)
Sample (Falling)
Setup (Falling)
Trailing edge
Setup (Falling)
Sample (Falling)
Setup (Rising)
Sample (Rising)
SPI mode
CPOL = 0, CPHA = 0
CPOL = 0, CPHA = 1
CPOL = 1, CPHA = 0
CPOL = 1, CPHA = 1
0
1
2
3
Figure 77. SPI Transfer Format with CPHA = 0
SCK (CPOL = 0)
mode 0
SCK (CPOL = 1)
mode 2
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SS
MSB first (DORD = 0) MSB
LSB first (DORD = 1) LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
Figure 78. SPI Transfer Format with CPHA = 1
SCK (CPOL = 0)
mode 1
SCK (CPOL = 1)
mode 3
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SS
MSB first (DORD = 0)
LSB first (DORD = 1)
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
170
ATmega128(L)
2467P–AVR–08/07
ATMEL [ ATMEL ]
