Aside from a couple of things back in the '80's (with MC68HC11) I've only used SPI to deal with relatively large serial memory a few years ago. That certainly had both commands/status to/from the device and data from master to/from slave. IIRC, it had to be told on which edge to operate as well, none of which was a terrible burden, but it certainly wasn't like communicating with one or more '595 or '597 registers.

I simply don't see the advantage of using SPI when one doesn't absolutely have to, hence, my rather narrow view.

Since I'm generally upgrading or expanding hardware that has been in place for decades, and doing so without respinning a PCB, I don't have the luxury of using an MCU that has dedicated SPI hardware, and have to make it, meaning the serial comm channel, whatever it is, a part of the memory-mapped I/O.

Maybe sometime I'll have to consider SPI for something specific, but I've so far managed to minimize my contact with it. Oddly enough, when I was using the 'HC11, there wasn't much hardware with which it could be used, as the feature was too new. Since I often use programmable logic (CPLD) for I/O, however, I probably will build a simple interface to family logic before I go to some SPI-specific hardware.

BTW, I didn't spend a lot of time with the WIKI article, but did scan it to ensure it covered most of the details for Per.

RE