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2545T–AVR–05/11
ATmega48/88/168
Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR Register is set, an interrupt
is requested. The Slave may continue to place new data to be sent into SPDR before reading
the incoming data. The last incoming byte will be kept in the Buffer Register for later use.
Figure 19-2. SPI master-slave interconnection.
The system is single buffered in the transmit direction and double buffered in the receive direc-
tion. This means that bytes to be transmitted cannot be written to the SPI Data Register before
the entire shift cycle is completed. When receiving data, however, a received character must be
read from the SPI Data Register before the next character has been completely shifted in. Oth-
erwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure
correct sampling of the clock signal, the minimum low and high periods should be:
Low periods: Longer than 2 CPU clock cycles.
High periods: Longer than 2 CPU clock cycles.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden
Note:
tion of the user defined SPI pins.
The following code examples show how to initialize the SPI as a Master and how to perform a
simple transmission. DDR_SPI in the examples must be replaced by the actual Data Direction
Register controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the
actual data direction bits for these pins. For example if MOSI is placed on pin PB3, replace
DD_MOSI with DDB3 and DDR_SPI with DDRB.
Table 19-1.
Pin
Direction, master SPI
Direction, slave SPI
MOSI
User defined
Input
MISO
Input
User defined
SCK
User defined
Input
SS
User defined
Input
SHIFT
ENABLE