SERIAL PERIPHERAL INTERFACE (SPI)
SERIAL PERIPHERAL INTERFACE (Cont’d)
10.7.4.7 Single Master and Multimaster Configurations
There are two types of SPI systems:
– Single Master System
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 its SPDR reg-
ister.
– Multimaster System
Single Master System
Other transmission security methods can use
ports for handshake lines or data bytes with com-
mand fields.
A typical single master system may be configured,
using an MCU as the master and four MCUs as
slaves (see Figure 125).
Multi-Master System
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.
A multi-master system may also be configured by
the user. Transfer of master control could be im-
plemented using a handshake method through the
I/O ports or by an exchange of code messages
through the serial peripheral interface system.
The SS pins are pulled high during reset since the
master device ports will be forced to be inputs at
that time, thus disabling the slave devices.
The multi-master system is principally handled by
the MSTR bit in the SPCR register and the MODF
bit in the SPSR register.
Note: To prevent a bus conflict on the MISO line
the master allows only one slave device during a
transmission.
Figure 125. Single Master Configuration
SS
SS
SS
SS
SCK
SCK
Slave
SCK
Slave
SCK
Slave
Slave
MCU
MCU
MCU
MCU
MOSI MISO
MOSI MISO
MOSI MISO
MOSI MISO
MOSI MISO
SCK
Master
MCU
5V
SS
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STMICROELECTRONICS [ ST ]
