4 ms·

Thanks for the detailed reply! I’m trying to understand where the roadblock on a rPi + small FPGA hybrid board for $50 fails at the task… and it sounds like th

Thanks for the detailed reply! I’m trying to understand where the roadblock on a rPi + small FPGA hybrid board for $50 fails at the task… and it sounds like the OS/firmware doesn’t suffice. (Or a SoC, like a Zynq.) Eg, if we could guarantee that the 1.5GHz core won’t “be off” by more than 1us on responding and the FPGA can manage IO directly to buffer out (some of) the jitter, then the cost of many hobby systems with “(still not quite) hard” real time systems would come down to reasonable.

rmu09 3y ago
You can get pretty far nowadays with preempt rt and an FPGA. Maybe you even can get near 1µs max jitter. One problem with the older RPis was unpredictable (to me) behaviour of the hardware, i.e. "randomly" changing SPI clocks, and limited bandwidth. Hobby systems like a small CNC mill or lathe usually don't need anything near 1µs (or better) max jitter. LinuxCNC (derived from NIST's Enhanced Machine Controller, name changed due to legal threats) runs fine on preempt-rt with control loops around 1kHz, with some systems you can also run a "fast" thread with say 20kHz and more to generate stepper motor signals, but that job is best left for the FPGA or an additional µC IMHO.