ST92F124 datasheet(253/429 Pages) STMICROELECTRONICS

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SERIAL PERIPHERAL INTERFACE (SPI)

SERIAL PERIPHERAL INTERFACE (Cont’d)

10.7.4.2 Slave Configuration

In slave configuration, the serial clock is received

on the SCK pin from the master device.

The value of the SPPR register and SPR0 & SPR1

bits in the SPCR is not used for the data transfer.

Procedure

– For correct data transfer, the slave device

must be in the same timing mode as the mas-

ter device (CPOL and CPHA bits). See Figure

123.

–The SS pin must be connected to a low level

signal during the complete byte transmit se-

quence.

– Clear the MSTR bit and set the SPOE bit to

assign the pins to alternate function.

In this configuration the MOSI pin is a data input

and the MISO pin is a data output.

Transmit Sequence

The data byte is parallel loaded into the 8-bit shift

and then shifted out serially to the MISO pin most

significant bit first.

The transmit sequence begins when the slave de-

vice receives the clock signal and the most signifi-

cant bit of the data on its MOSI pin.

When data transfer is complete:

– The SPIF bit is set by hardware

– An interrupt is generated if the SPIS and SPIE

bits are set.

During the last clock cycle the SPIF bit is set, a

copy of the data byte received in the shift register

is moved to a buffer. When the SPDR register is

read, the SPI peripheral returns this buffered val-

ue.

Clearing the SPIF bit is performed by the following

software sequence:

1. An access to the SPSR register while the SPIF

bit is set.

2. A read of the SPDR register.

Notes: While the SPIF bit is set, all writes to the

SPDR register are inhibited until the SPSR regis-

ter is read.

The SPIF bit can be cleared during a second

transmission; however, it must be cleared before

the second SPIF bit in order to prevent an overrun

condition (see Section 10.7.4.6).

Depending on the CPHA bit, the SS pin has to be

set to write to the SPDR register between each

data byte transfer to avoid a write collision (see

Section 10.7.4.4).

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