Datasheet

Serial Peripheral Interface Module (SPI)

Technical Data MC68HC908LJ12 — Rev. 2.1

282 Serial Peripheral Interface Module (SPI) Freescale Semiconductor

For an idle master or idle slave that has no data loaded into its transmit

buffer, the SPTE is set again no more than two bus cycles after the

transmit buffer empties into the shift register. This allows the user to

queue up a 16-bit value to send. For an already active slave, the load of

the shift register cannot occur until the transmission is completed. This

implies that a back-to-back write to the transmit data register is not

possible. The SPTE indicates when the next write can occur.

14.8 Error Conditions

The following flags signal SPI error conditions:

• 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

• 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.

14.8.1 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 Figure 14-4 and Figure 14-6.) If

an overflow occurs, all data 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

OVRF generates a receiver/error CPU interrupt request if the error

interrupt enable bit (ERRIE) is also set. The SPRF, MODF, and OVRF