Datasheet

90
AT89C51AC3
4383D–8051–02/08
Figure 51. Mode Fault Conditions in Slave Mode
Note: when SS is discarded (SS disabled) it is not possible to detect a MODF error in slave
mode because the SPI is internally selected. Also the SS pin becomes a general pur-
pose I/O.
OverRun Condition This error mean that the speed is not adapted for the running application:
An OverRun condition occurs when a byte has been received whereas the previous one
has not been read by the application yet.
The last byte (which generate the overrun error) does not overwrite the unread data so
that it can still be read. Therefore, an overrun error always indicates the loss of data.
Interrupts Three 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 request only when
SPTEIE is disabled.
Mode Fault flag, MODF: This bit is set to indicate that the level on the SS is inconsistent
with the mode of the SPI (in both master and slave modes).
Serial Peripheral Transmit Register empty flag, SPTE: This bit is set when the transmit
buffer is empty (other data can be loaded is SPDAT). SPTE bit generates transmitter
CPU interrupt request only when SPTEIE is enabled.
Note: While using SPTE interruption for “burst mode” transfers (SPTEIE=’1’), the
user software application should take care to clear SPTEIE, during the last but one
data reception (to be able to generate an interrupt on SPIF flag at the end of the last
data reception).
SCK
1 2 3 SCK cycle # 0
SS
(slave)
(from master)
MODF detected
B6MSB
B6MSB
0
z
1
0
z
1
0
z
1
0
z
1
0
MODF detected
MOSI
MISO
(from master)
(from slave)
MSB
B5 B4
4
Flag Request
SPIF (SPI data transfer) SPI Transmitter Interrupt Request
MODF (Mode Fault) SPI mode-fault Interrupt Request
SPTE (Transmit register empty) SPI transmit register empty Interrupt Request