Something seems weird about this interface.

The datasheets that I have looked at seem to say that the hardware SPI is only capable of communicating in 8 bit bytes. So even if you were able to write a program that handled the HWSPI, you would be locked into devices that used 8 bits.
I seems that so many of the desired devices that use SPI require more than that.

The above D/A is one example. An SCA61T inclinometer that I'm playing with is using 11 bit data.

I'm going to read the inclinometer and output to the D/A so it looks like a software SPI is the ONLY solution. I intent to try a variation on Bruce's multi-bit write above to do similar with the read so we'll see how that goes.

Sure seems odd that is the case if I understand it correctly.

Bo