Serial Peripheral Interface Module (SPI)
BYTE 1
BYTE 2
5
BYTE 3
7
BYTE 4
11
SPI RECEIVE
COMPLETE
1
SPRF
OVRF
READ
SPSCR
2
4
6
9
12
14
READ
SPDR
3
8
10
13
1
2
8
9
BYTE 1 SETS SPRF BIT.
CPU READS BYTE 2 IN SPDR,
CLEARING SPRF BIT.
CPU READS SPSCR WITH SPRF BIT SET
AND OVRF BIT CLEAR.
CPU READS SPSCR AGAIN
TO CHECK OVRF BIT.
3
4
CPU READS BYTE 1 IN SPDR,
CLEARING SPRF BIT.
10
CPU READS BYTE 2 SPDR,
CLEARING OVRF BIT.
CPU READS SPSCR AGAIN
TO CHECK OVRF BIT.
11
12
13
BYTE 4 SETS SPRF BIT.
CPU READS SPSCR.
5
6
BYTE 2 SETS SPRF BIT.
CPU READS SPSCR WITH SPRF BIT SET
AND OVRF BIT CLEAR.
CPU READS BYTE 4 IN SPDR,
CLEARING SPRF BIT.
7
BYTE 3 SETS OVRF BIT. BYTE 3 IS LOST.
14
CPU READS SPSCR AGAIN
TO CHECK OVRF BIT.
Figure 15-10. Clearing SPRF When OVRF Interrupt Is Not Enabled
15.6.2 Mode Fault Error
Setting the SPMSTR bit selects master mode and configures the SPSCK and
MOSI pins as outputs and the MISO pin as an input. Clearing SPMSTR selects
slave mode and configures the SPSCK and MOSI pins as inputs and the MISO pin
as an output. The mode fault bit, MODF, becomes set any time the state of the
slave select pin, SS, is inconsistent with the mode selected by SPMSTR.
To prevent SPI pin contention and damage to the MCU, a mode fault error occurs if:
•
•
The SS pin of a slave SPI goes high during a transmission.
The SS pin of a master SPI goes low at any time.
For the MODF flag to be set, the mode fault error enable bit (MODFEN) must be
set. Clearing the MODFEN bit does not clear the MODF flag but does prevent
MODF from being set again after MODF is cleared.
MODF generates a receiver/error CPU interrupt request if the error interrupt enable
bit (ERRIE) is also set. The SPRF, MODF, and OVRF interrupts share the same
CPU interrupt vector. MODF and OVRF can generate a receiver/error CPU
interrupt request. See Figure 15-11. It is not possible to enable MODF or OVRF
individually to generate a receiver/error CPU interrupt request. However, leaving
MODFEN low prevents MODF from being set.
Data Sheet
220
MC68HC908MR32 • MC68HC908MR16 — Rev. 6.0
Serial Peripheral Interface Module (SPI)
MOTOROLA
FREESCALE [ Freescale ]
