This little project <https://github.com/cariboulabs/cariboulite> for an SDR (Software-Defined Radio) addon card for a Raspberry Pi popped up on my feed. What caught my eye was that it made use of something called “SMI” (the “Secondary Memory Interface”) which exists on the GPIO connector but is not properly documented. There is a link to this blog post <https://iosoft.blog/2020/07/16/raspberry-pi-smi/> in which the writer reports some investigations into the details of this interface. Speed tests in that article give results of 25 16-bit samples per second or more, depending on the Raspberry Pi model; though note there are some odd glitches sometimes, as well as high bus contention (clearly pushing the Pi hardware to its limits). Maybe this is why the interface is not documented: because it is not considered production-ready ...