AT89C51ED2-RDTUM datasheet(68/137 Pages) ATMEL

68 / 137 page

4235K–8051–05/08

AT89C51RD2/ED2

16.3.3.1

Mode Fault (MODF)

Mode Fault error in Master mode SPI indicates that the level on the Slave Select (SS) pin is

inconsistent with the actual mode of the device. MODF is set to warn that there may be a multi-

master conflict for system control. In this case, the SPI system is affected in the following ways:

• An SPI receiver/error CPU interrupt request is generated

• The SPEN bit in SPCON is cleared. This disables the SPI

• The MSTR bit in SPCON is cleared

When SS Disable (SSDIS) bit in the SPCON register is cleared, the MODF flag is set when the

SS signal becomes ’0’.

However, as stated before, for a system with one Master, if the SS pin of the Master device is

pulled low, there is no way that another Master attempts to drive the network. In this case, to

prevent the MODF flag from being set, software can set the SSDIS bit in the SPCON register

and therefore making the SS pin as a general-purpose I/O pin.

Clearing the MODF bit is accomplished by a read of SPSTA register with MODF bit set, followed

by a write to the SPCON register. SPEN Control bit may be restored to its original set state after

the MODF bit has been cleared.

16.3.3.2

Write Collision (WCOL)

A Write Collision (WCOL) flag in the SPSTA is set when a write to the SPDAT register is done

during a transmit sequence.

WCOL does not cause an interruption, and the transfer continues uninterrupted.

Clearing the WCOL bit is done through a software sequence of an access to SPSTA and an

access to SPDAT.

16.3.3.3

Overrun Condition

An overrun condition occurs when the Master device tries to send several data Bytes and the

Slave devise 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 of the

SPDAT returns this Byte. All others Bytes are lost.

This condition is not detected by the SPI peripheral.

16.3.3.4

SS Error Flag (SSERR)

A Synchronous Serial Slave Error occurs when SS goes high before the end of a received data

in slave mode. SSERR does not cause in interruption, this bit is cleared by writing 0 to SPEN bit

(reset of the SPI state machine).

16.3.4

Interrupts

Two SPI status flags can generate a CPU interrupt requests:

Table 16-2.

SPI Interrupts

Flag

Request

SPIF (SP data transfer)

SPI Transmitter Interrupt request

MODF (Mode Fault)

SPI Receiver/Error Interrupt Request (if SSDIS = ’0’)

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