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Rough plan of experimentation/implementation:

✅ = done, 📝 = work in progress, 💭 = still to be done

✅ Design prototype PCB, with level shifters and lines for IRQ and DMA
✅ Source parts & build prototype PCB
✅ Run RPiISA code on Linux on prototype PCB, validating level shifters, changing design/bodging as needed
✅ Port RPiISA code to Circle, using PWM audio out
📝 Extract GUS emulation code from DOSBox
💭 Implement port-only emulation
💭 Implement IRQs
💭 Implement sample upload via DMA
💭 Investigate moving GPIOs to free up I2S pins for better audio output quality via external DAC
💭 Design release candidate PCB

Known issues

• The GPIO header does not give enough clearance for the Raspberry Pi when plugged directly in via straight headers. A ribbon cable must be used instead, or a pair of right-angle headers
• There might not be enough GPIO pins to handle proper DMA
• The I2S GPIO pins are used, preventing the use of a high quality external DAC