MC68HC908GZ8 Datasheet, PDF (285/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Serial Peripheral Interface (SPI) Module
Error Conditions
BYTE 1
1
BYTE 2
4
BYTE 3
6
BYTE 4
8
SPRF
OVRF
READ
2
5
SPSCR
READ
SPDR
3
7
1 BYTE 1 SETS SPRF BIT.
2 CPU READS SPSCR WITH SPRF BIT SET
AND OVRF BIT CLEAR.
3 CPU READS BYTE 1 IN SPDR,
CLEARING SPRF BIT.
4 BYTE 2 SETS SPRF BIT.
5 CPU READS SPSCR WITH SPRF BIT SET
AND OVRF BIT CLEAR.
6 BYTE 3 SETS OVRF BIT. BYTE 3 IS LOST.
7 CPU READS BYTE 2 IN SPDR, CLEARING SPRF BIT,
BUT NOT OVRF BIT.
8 BYTE 4 FAILS TO SET SPRF BIT BECAUSE
OVRF BIT IS NOT CLEARED. BYTE 4 IS LOST.
Figure 21-9. Missed Read of Overflow Condition
In this case, an overflow can be missed easily. Since no more SPRF interrupts can
be generated until this OVRF is serviced, it is not obvious that bytes are being lost
as more transmissions are completed. To prevent this, either enable the OVRF
interrupt or do another read of the SPSCR following the read of the SPDR. This
ensures that the OVRF was not set before the SPRF was cleared and that future
transmissions can set the SPRF bit. Figure 21-10 illustrates this process.
Generally, to avoid this second SPSCR read, enable the OVRF to the CPU by
setting the ERRIE bit.
21.7.2 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.
MC68HC908GZ8
MOTOROLA
Serial Peripheral Interface (SPI) Module
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Data Sheet
285