ST7L09 datasheet(59/104 Pages) STMICROELECTRONICS

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ST7L05, ST7L09

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SERIAL PERIPHERAL INTERFACE (Cont’d)

11.3.5.4 Single Master Systems

A typical single master system may be configured,

using an MCU as the master and four MCUs as

slaves (see Figure 37).

The master device selects the individual slave de-

vices by using four pins of a parallel port to control

the four SS pins of the slave devices.

The SS pins are pulled high during reset since the

master device ports are forced as inputs at that

time, thus disabling the slave devices.

Note: To prevent a bus conflict on the MISO line

the master allows only one active slave device

during a transmission.

For more security, the slave device may respond

to the master with the received data byte. Then the

master will receive the previous byte back from the

slave device if all MISO and MOSI pins are con-

nected and the slave has not written to its SPIDR

Other transmission security methods can use

ports for handshake lines or data bytes with com-

mand fields.

Figure 37. Single Master / Multiple Slave Configuration

MISO

MOSI

MISO

SCK

Slave

MCU

Slave

MCU

Slave

MCU

Slave

MCU

Master

MCU

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