AT89C51ED2-RDTIM datasheet(69/137 Pages) ATMEL

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AT89C51RD2/ED2

4235E–8051–04/04

Error Conditions

The following flags in the SPSTA signal SPI error conditions:

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 orig-

inal set state after the MODF bit has been cleared.

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.

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.

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).

Interrupts

Two SPI status flags can generate a CPU interrupt requests:

Table 47. SPI Interrupts

Serial Peripheral data transfer flag, SPIF: This bit is set by hardware when a transfer

has been completed. SPIF bit generates transmitter CPU interrupt requests.

Mode Fault flag, MODF: This bit becomes set to indicate that the level on the SS is

inconsistent with the mode of the SPI. MODF with SSDIS reset, generates receiver/error

CPU interrupt requests. When SSDIS is set, no MODF interrupt request is generated.

Figure 35 gives a logical view of the above statements.

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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