MC68HC908AZ60 Datasheet, PDF (252/480 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Serial Peripheral Interface Module (SPI)
BYTE 1
1
BYTE 2
4
BYTE 3
6
BYTE 4
8
SPRF
OVRF
READ SPSCR
2
5
READ SPDR
3
7
1 BYTE 1 SETS SPRF BIT.
5 CPU READS SPSCRW WITH SPRF BIT SET
2 CPU READS SPSCR WITH SPRF BIT SET ^{AND OVRF BIT CLEAR.}
AND OVRF BIT CLEAR.
6 BYTE 3 SETS OVRF BIT. BYTE 3 IS LOST.
3 CPU READS BYTE 1 IN SPDR,
CLEARING SPRF BIT.
4 BYTE 2 SETS SPRF BIT.
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 SET. BYTE 4 IS LOST.
Figure 6. Missed Read of Overflow Condition
The first part of Figure 6 shows how to read the SPSCR and SPDR to
clear the SPRF without problems. However, as illustrated by the second
transmission example, the OVRF flag can be set in between the time
that SPSCR and SPDR are read.
In this case, an overflow can be easily missed. Since no more SPRF
interrupts can be generated until this OVRF is serviced, it will not be
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 after the read of the SPDR. This ensures that the OVRF was
not set before the SPRF was cleared and that future transmissions will
complete with an SPRF interrupt. Figure 7 illustrates this process.
Generally, to avoid this second SPSCR read, enable the OVRF to the
CPU by setting the ERRIE bit (SPSCR).
MC68HC908AZ60 — Rev 2.0
250
Serial Peripheral Interface Module (SPI)
14-spi
MOTOROLA