ST72321 datasheet(92/185 Pages) STMICROELECTRONICS

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ST72321

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SERIAL PERIPHERAL INTERFACE (Cont’d)

10.5.4 Clock Phase and Clock Polarity

Four possible timing relationships may be chosen

by software, using the CPOL and CPHA bits (See

Figure 57).

Note: 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 57, shows an SPI transfer with the four

combinations of the CPHA and CPOL bits. The di-

agram may be interpreted as a master or slave

timing diagram where the SCK pin, the MISO pin,

the MOSI pin are directly connected between the

master and the slave device.

Note: If CPOL is changed at the communication

byte boundaries, the SPI must be disabled by re-

setting the SPE bit.

Figure 57. Data Clock Timing Diagram

SCK

MSBit

Bit 6

Bit 5

Bit 4

Bit3

Bit 2

Bit 1

LSBit

MSBit

Bit 6

Bit 5

Bit 4

Bit3

Bit 2

Bit 1

LSBit

MISO

(from master)

MOSI

(from slave)

(to slave)

CAPTURE STROBE

CPHA =1

MSBit

Bit 6

Bit 5

Bit 4

Bit3

Bit 2

Bit 1

LSBit

MSBit

Bit 6

Bit 5

Bit 4

Bit3

Bit 2

Bit 1

LSBit

MISO

(from master)

MOSI

(to slave)

CAPTURE STROBE

CPHA =0

Note: This figure should not be used as a replacement for parametric information.

Refer to the Electrical Characteristics chapter.

(from slave)

(CPOL = 1)

SCK

(CPOL = 0)

SCK

(CPOL = 1)

SCK

(CPOL = 0)

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