ST92F124 datasheet(257/429 Pages) STMICROELECTRONICS

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SERIAL PERIPHERAL INTERFACE (SPI)

SERIAL PERIPHERAL INTERFACE (Cont’d)

10.7.4.5 Master Mode Fault

Master mode fault occurs when the master device

has its SS pin pulled low, then the MODF bit is set.

Master mode fault affects the SPI peripheral in the

following ways:

– The MODF bit is set and an SPI interrupt is

generated if the SPIE bit is set.

– The SPOE bit is reset. This blocks all output

from the device and disables the SPI periph-

eral.

– The MSTR bit is reset, thus forcing the device

into slave mode.

Clearing the MODF bit is done through a software

sequence:

1. A read access to the SPSR register while the

MODF bit is set.

2. A write to the SPCR register.

Notes: To avoid any multiple slave conflicts in the

case of a system comprising several MCUs, the

SS pin must be pulled high during the clearing se-

quence of the MODF bit. The SPOE and MSTR

bits may be restored to their original state during or

after this clearing sequence.

Hardware does not allow the user to set the SPOE

and MSTR bits while the MODF bit is set except in

the MODF bit clearing sequence.

In a slave device the MODF bit can not be set, but

in a multi master configuration the device can be in

slave mode with this MODF bit set.

The MODF bit indicates that there might have

been a multi-master conflict for system control and

allows a proper exit from system operation to a re-

set or default system state using an interrupt rou-

tine.

10.7.4.6 Overrun Condition

An overrun condition occurs, when the master de-

vice has sent several data bytes and the slave de-

vice has not cleared the SPIF bit issuing from the

previous data byte transmitted.

In this case, the receiver buffer contains the byte

sent after the SPIF bit was last cleared. A read to

the SPDR register returns this byte. All other bytes

are lost.

This condition is not detected by the SPI peripher-

al.

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