65
ATmega161(L)
1228B–09/01
tem interprets this as another master selecting the SPI as a slave and starting to send
data to it. To avoid bus contention, the SPI system takes the following actions:
1.
The MSTR bit in SPCR is cleared and the SPI system becomes a slave. As a
result of the SPI becoming a slave, the MOSI and SCK pins become inputs.
2.
The SPIF flag in SPSR is set, and if the SPI interrupt is enabled and the I-bit in
SREG is set, the interrupt routine will be executed.
Thus, when interrupt-driven SPI transmission is used in master mode, and there exists a
possibility that SS is driven low. The interrupt should always check that the MSTR bit is
still set. Once the MSTR bit has been cleared by a slave select, it must be set by the
user to re-enable SPI master mode.
When the SPI is configured as a slave, the SS pin is always input. When SS is held low,
the SPI is activated and MISO becomes an output if configured so by the user. All other
pins are inputs. When SS is driven high, all pins are inputs and the SPI is passive, which
means that it will not receive incoming data. Note that the SPI logic will be reset once
the SS pin is brought high. If the SS pin is brought high during a transmission, the SPI
will stop sending and receiving immediately and both data received and data sent must
be considered lost.
Data Modes
There are four combinations of SCK phase and polarity with respect to serial data,
which are determined by control bits CPHA and CPOL. The SPI data transfer formats
are shown in Figure 43 and Figure 44.
Figure 43.
SPI Transfer Format with CPHA = 0 and DORD = 0
Figure 44.
SPI Transfer Format with CPHA = 1 and DORD = 0