Basically the title, but I have been looking into SPI as I am working on a project that basically requires a Serial connection between a Raspberry Pi (as a master) and an Arduino (slave); I intended on having the Pi act as a master but still have the Arduino freely communicate back to the Pi (communication such as notification of errors or completion of a task, etc.)
Though my research in this SPI thing has been a little conflicting because one source either says it is a must that I use logic level converters, and others just do a direct connection. Which would it be? I only plan on using one Raspberry Pi and one Arduino, the Arduino in which sends updates at how machinery is running and requests to enable that machinery, along with the Pi receiving the data and sending commands on what to do.