This is the official firmware for the Octasonic HC-SR04 Breakout that allows up to 8 HC-SR04 ultrasonic sensors to be monitored via SPI.
Make sure you have an appropriate AVR toolchain installed, following these instructions.
You will also need an AVR programmer to connect to the Octasonic board. I use this one.
Connect the Octasonic board to the AVR Programmer and connect the AVR Programmer to a USB port on your computer and run the following commands.
cd firmware make clean make flash make fuses