Firmware SpiSlv is used to enable the SPI port on a RPUno (ATmega328p).

After loading the firmware on an RPUno and enabling the AVR's SPI I next need the Raspberry Pi SPI hardware setup. Raspian needs its interface master driver enabled (e.g. raspi-config).

Raspibin has an spi group setup in /etc/udev/rules.d/99-com.rules. So I can add my user name to that group for the system to allow me to use the interface.

sudo usermod -a -G spi rsutherland
# logout for the change to take

Next I compile and run spidev_test.c on the Pi with:

wget https://raw.githubusercontent.com/raspberrypi/linux/rpi-3.10.y/Documentation/spi/spidev_test.c
gcc -o spidev_test spidev_test.c
# run with
./spidev_test -s 100000 -D /dev/spidev0.0
./spidev_test -s 500000 -D /dev/spidev0.0
./spidev_test -s 1000000 -D /dev/spidev0.0
./spidev_test -s 2000000 -D /dev/spidev0.0
spi mode: 0
bits per word: 8
max speed: 500000 Hz (500 KHz)
0D FF FF FF FF FF
FF 40 00 00 00 00
95 FF FF FF FF FF
FF FF FF FF FF FF
FF FF FF FF FF FF
FF DE AD BE EF BA
AD F0

Note: The output is offset a byte since it was sent back from the AVR. I see errors at 1MHz and 2MHz. The RPUpi^3 has a 74LVC07 buffer that pulls down through a 10kOhm resistor on SCK and MOSI. The circuit between that resistor and the AVR has some capacitance which will limit the speed (if there is a need for higher speed it may be possible to swap in a 1k resistor).