Is this project still being worked on? This seems like it would be more expandable than the Genesis Forever project. I also saw in a YouTube comment you left on one of your videos (for Genesis Forever though) that you planned on putting kits together for this, I would definitely be interested in that.

I really enjoyed your demo on the HAD TV hangout earlier.

Here's a thought on your timing problems in Linux: Let the SPI interface do the timing for you.

Your operations are (I think) all writes, so use the SPI master in DMA mode and keep the buffer full with bus operations.  You would set the SPI bus speed such that a full scan of the shift registers takes less than the minimum state change timing on the parallel bus.  This would be your bus timing quantum.  You would then repeat the same value to the shift registers for output N times to create a delay of N * shift_register_bits * 1/SPI_clock.  Writing the same value to the shift registers is a no-op of course.

This would eliminate dependency on sleep() to keep time, and the DMA buffer would let your task ride out gaps where it gets rescheduled.

Thanks :) it was fun

that's a really clever strategy! I'll read up more on DMA to figure out how to do this kind of wizardry. do you know of any code online that you could point me towards for a good example?

i haven't used it myself, but I did find this fork of the existing SPI driver: https://github.com/notro/spi-bcm2708/wiki

In order to use it you'd have to write a kernel driver.  But maybe having your parallel bus abstraction in the kernel would be better.  You could create a character device interface that takes address/data to write to the bus along with a delay, and the driver fills the DMA buffer with the proper number of SPI shift cycles to meet the delay time.  Then you could eliminate your C user space driver altogether.

I recently obtained a SID from a C64, would be interesting to connect that to a PI. Thanks for the inspiration (if I'll ever get to do that)

which chip is that? is it easy to get more :) I bet starting a C64 branch would bring a lot of people in!

Well I got the 8580R5 out of it, but there's also the 6581 - https://www.c64-wiki.com/index.php/SID - they're a bit pricey these days. But there's a big fanbase..