ST72321B datasheet(89/187 Pages) STMICROELECTRONICS

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SERIAL PERIPHERAL INTERFACE (Cont’d)

–SS: Slave select:

This input signal acts as a ‘chip select’ to let

the SPI master communicate with slaves indi-

vidually and to avoid contention on the data

lines. Slave SS inputs can be driven by stand-

ard I/O ports on the master MCU.

10.5.3.1 Functional Description

A basic example of interconnections between a

single master and a single slave is illustrated in

Figure 55.

The MOSI pins are connected together and the

MISO pins are connected together. In this way

data is transferred serially between master and

slave (most significant bit first).

The communication is always initiated by the mas-

ter. When the master device transmits data to a

slave device via MOSI pin, the slave device re-

sponds by sending data to the master device via

the MISO pin. This implies full duplex communica-

tion with both data out and data in synchronized

with the same clock signal (which is provided by

the master device via the SCK pin).

To use a single data line, the MISO and MOSI pins

must be connected at each node ( in this case only

simplex communication is possible).

Four possible data/clock timing relationships may

be chosen (see Figure 58) but master and slave

must be programmed with the same timing mode.

Figure 55. Single Master/ Single Slave Application

8-BIT SHIFT REGISTER

SPI

CLOCK

GENERATOR

8-BIT SHIFT REGISTER

MISO

MOSI

MISO

SCK

SLAVE

MASTER

+5V

MSBit

LSBit

MSBit

LSBit

Not used if SS is managed

by software

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