s908az32ag2cfue Freescale Semiconductor, Inc, s908az32ag2cfue Datasheet - Page 173

• Manufacturer Part Number: s908az32ag2cfue
• Description: M68hc08 Microcontrollers 8-bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

17.6 Error Conditions
Two flags signal SPI error conditions:
17.6.1 Overflow Error
The overflow flag (OVRF in SPSCR) becomes set if the SPI receive data register still has unread data
from a previous transmission when the capture strobe of bit 1 of the next transmission occurs. (See
17-4
register so that the unread data can still be read. Therefore, an overflow error always indicates the loss
of data.
OVRF generates a receiver/error CPU interrupt request if the error interrupt enable bit (ERRIE in SPSCR)
is also set. MODF and OVRF can generate a receiver/error CPU interrupt request. (See
is not possible to enable only MODF or OVRF to generate a receiver/error CPU interrupt request.
However, leaving MODFEN low prevents MODF from being set.
If an end-of-block transmission interrupt was meant to pull the MCU out of wait, having an overflow
condition without overflow interrupts enabled causes the MCU to hang in wait mode. If the OVRF is
enabled to generate an interrupt, it can pull the MCU out of wait mode instead.
If the CPU SPRF interrupt is enabled and the OVRF interrupt is not, watch for an overflow condition.
Figure 17-7
Freescale Semiconductor
1. Overflow (OVRF in SPSCR) — Failing to read the SPI data register before the next byte enters the
2. Mode fault error (MODF in SPSCR) — The MODF bit indicates that the voltage on the slave select
and
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 by accessing the SPI data register. OVRF is in the SPI status and
control register.
pin (SS) is inconsistent with the mode of the SPI. MODF is in the SPI status and control register.
Figure
shows how it is possible to miss an overflow.
READ SPSCR
READ SPDR
17-5.) If an overflow occurs, the data being received is not transferred to the receive data
OVRF
SPRF
1
2
3
4
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.
Figure 17-7. Missed Read of Overflow Condition
1
2
MC68HC908AZ32A Data Sheet, Rev. 2
3
BYTE 2
4
5
6
7
8
BYTE 3
5
CPU READS SPSCRW 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 SET. BYTE 4 IS LOST.
6
7
BYTE 4
8
Figure
Error Conditions
17-9). It
Figure
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