ST72324BJ2 datasheet(94/188 Pages) STMICROELECTRONICS

94 / 188 page

On-chip peripherals

ST72324B

94/188

Figure 52.

Generic SS timing diagram

Figure 53.

Hardware/software slave select management

Master mode operation

In master mode, the serial clock is output on the SCK pin. The clock frequency, polarity and

phase are configured by software (refer to the description of the SPICSR register).

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).

How to operate the SPI in master mode

To operate the SPI in master mode, perform the following steps in order:

Write to the SPICR register:

–

Select the clock frequency by configuring the SPR[2:0] bits.

–

Select the clock polarity and clock phase by configuring the CPOL and CPHA bits.

Figure 54 shows the four possible configurations.

Note: The slave must have the same CPOL and CPHA settings as the master.

Write to the SPICSR register:

–

Either set the SSM bit and set the SSI bit or clear the SSM bit and tie the SS pin

high for the complete byte transmit sequence.

Write to the SPICR register:

–

Set the MSTR and SPE bits.

Note: MSTR and SPE bits remain set only if SS is high.

Caution:

If the SPICSR register is not written first, the SPICR register setting (MSTR bit) might not be

taken into account.

The transmit sequence begins when software writes a byte in the SPIDR register.

Byte 1

Byte 2

Byte 3

MOSI/MISO

Master SS

Slave SS

(if CPHA=0)

Slave SS

(if CPHA=1)

SS internal

SSM bit

SSI bi

SS external pin

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