MC68HC08AS32 Datasheet, PDF (288/394 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

Serial Peripheral Interface (SPI)
18.7 Error Conditions
Two flags signal SPI error conditions:
1. Overflow (OVRFin SPSCR) — Failing to read the SPI data register
before the next 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 by accessing the SPI data
register. OVRF is in the SPI status and control register.
2. Mode fault error (MODF in SPSCR) — 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.
18.7.1 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 Figure 18-3
and Figure 18-5.) If an overflow occurs, the data being received is not
transferred to the receive data 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 Figure 18-9.) It 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 18-7 shows how it is possible to
miss an overflow.
Advance Information
288
Serial Peripheral Interface (SPI)
MC68HC08AS32 — Rev. 3.0
MOTOROLA