Pi:Devices
NextPi Communication Devices
Canonical version of this document lives on GitLab
There is no hardware connectivity between the Next and the Pi outside protocols documented below, and no other existing hardware control points exist between the two devices at this time. While is is possible that future connections between the two endpoints could be created, DO NOT presume this is the case as the FPGA on the original KS1 Nexts is very close to capacity already.
You can build your own custom communications protocol, but that is left as an exercise to the developer. "No hardware link outside of these connections" between the two systems means no direct memory, no physical device access, etc. No integrated instructions on the CPU, or hardware beyond that mentioned below.
That said, a developer could build a routine over GPIO to sit on the Pi side, and treat the Pi as a "Copper", or to handle large memory transfers, etc. but they need to start from "foundations" for all that.
UART
The UART currently runs at 115,000 and is paired to a BASH Supervisor shell, running at ROOT, with a preconfigured path to support the default CLI override system. This is how Pisend, Piput, Piget, etc. (current NextZXOS integrations) all work. File-transfers and "terminal commands" between the pi and Next are currently handled via this interface.
I2S DAC
The ALSA stack is configured to emulate a TOSLink interface, sending digital PCMdata to the Next for it to decode upon its own DAC natively. This means that any API that natively supports ALSA on linux gets access to the next output DAC. Because this DAC is a "pure digital" solution, with no DSP attached, volume control needs to be done at the sample generation/output level, upon the pi, or likewise upon the next itself - there is no "volume control" or "amp" between the two systems. Audio players, including TZX loading, are currently handled via this interface.
GPIO
The Next's own GPIO lines are connected to the Pi header, and therefore permit the creation of custom GPIO based protocols for communication and control. This isn't currently used by any included applications in NextZXOS, but has been proven to be a practical method for those who wish to adopt it.