MC68HC908GT16_07 FREESCALE [Freescale Semiconductor, Inc], MC68HC908GT16_07 Datasheet - Page 206

• Manufacturer Part Number: MC68HC908GT16_07
• Description: Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Serial Peripheral Interface (SPI) Module
16.6 Error Conditions
The following flags signal SPI error conditions:
16.6.1 Overflow Error
The overflow flag (OVRF) becomes set if the receive data register still has unread data from a previous
transmission when the capture strobe of bit 1 of the next transmission occurs. The bit 1 capture strobe
occurs in the middle of SPSCK cycle 7 (see
received after the overflow and before the OVRF bit is cleared does not transfer to the receive data
register and does not set the SPI receiver full bit (SPRF). The unread data that transferred to the receive
data register before the overflow occurred can still be read. Therefore, an overflow error always indicates
the loss of data. Clear the overflow flag by reading the SPI status and control register and then reading
the SPI data register.
OVRF 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 (see
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.
If the CPU SPRF interrupt is enabled and the OVRF interrupt is not, watch for an overflow condition.
Figure 16-10
possible to read the SPSCR and SPDR to clear the SPRF without problems. However, as illustrated by
the second transmission example, the OVRF bit can be set in between the time that SPSCR and SPDR
are read.
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Overflow (OVRF) — Failing to read the SPI data register before the next full byte enters the shift
register sets the OVRF bit. The new byte does not transfer to the receive data register, and the
unread byte still can be read. OVRF is in the SPI status and control register.
Mode fault error (MODF) — The MODF bit indicates that the voltage on the slave select pin (SS)
is inconsistent with the mode of the SPI. MODF is in the SPI status and control register.
SPSCR
shows how it is possible to miss an overflow. The first part of
OVRF
READ
READ
SPDR
SPRF
1
2
3
4
MC68HC908GT16 • MC68HC908GT8 • MC68HC08GT16 Data Sheet, Rev. 5.0
BYTE 1
BYTE 1 SETS SPRF BIT.
CPU READS SPSCR WITH SPRF BIT SET
AND OVRF BIT CLEAR.
CPU READS BYTE 1 IN SPDR,
CLEARING SPRF BIT.
BYTE 2 SETS SPRF BIT.
1
Figure 16-10. Missed Read of Overflow Condition
2
3
BYTE 2
4
Figure 16-5
BYTE 3
5
5
6
7
8
6
and
CPU READS SPSCR WITH SPRF BIT SET
AND OVRF BIT CLEAR.
BYTE 3 SETS OVRF BIT. BYTE 3 IS LOST.
CPU READS BYTE 2 IN SPDR, CLEARING SPRF BIT,
BUT NOT OVRF BIT.
BYTE 4 FAILS TO SET SPRF BIT BECAUSE
OVRF BIT IS NOT CLEARED. BYTE 4 IS LOST.
7
Figure
16-7.) If an overflow occurs, all data
BYTE 4
8
Figure 16-10
Freescale Semiconductor
Figure
shows how it is
16-12.) It