MC68HC08XL36 Datasheet, PDF (207/362 Pages) Freescale Semiconductor, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Serial Peripheral Interface Module (SPI)
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 10. Missed Read of Overflow Condition
In this case, an overflow can easily be missed. 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 11 illustrates this process. Generally, to
avoid this second SPSCR read, enable the OVRF to the CPU by setting
the ERRIE bit.
17-spi_c
MOTOROLA
Serial Peripheral Interface Module (SPI)
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MC68HC08XL36
207