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AT89S53

4-231

clock output in the master mode but is the clock input in the

slave mode. Writing to the SPI data register of the master

CPU starts the SPI clock generator, and the data written

shifts out of the MOSI pin and into the MOSI pin of the

slave CPU. After shifting one byte, the SPI clock generator

stops, setting the end of transmission flag (SPIF). If both

the SPI interrupt enable bit (SPIE) and the serial port inter-

rupt enable bit (ES) are set, an interrupt is requested.

The Slave Select input, SS

/P1.4, is set low to select an

individual SPI device as a slave. When SS

/P1.4 is set high,

the SPI port is deactivated and the MOSI/P1.5 pin can be

used as an input.

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 Figures 8 and 9.

Figure 7.

SPI Master-Slave Interconnection

8-BIT SHIFT REGISTER

MASTER

CLOCK GENERATOR

SPI

MISO

8-BIT SHIFT REGISTER

SLAVE

MISO

MOSI MOSI

SCK

SS SS

MSB LSB

Figure 8.

SPI transfer Format with CPHA = 0

*Not defined but normally MSB of character just received

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Atmel C51 Datenblatt Seite 17