Designing a new computer

Posted by pulkomandy on Wed Aug 30 18:22:24 2023  •  Comments (0)  • 

This project started by breaking old hardware. A few years ago, I started experimenting with designing hardware expansions for the Amstrad PCW. I had some success previously with the Amstrad CPC, surely this couldn't be so different? And it's a kind of interesting machine, with lots of RAM (for an 8bit system) and a quite nicely designes ASIC which would make programming it relatively nice.

My hardware experiments, however, did not go so well. First I got just a motherboard, and I hacked up a way to connect it to an Amstrad CTM and hooked a floppy drive. It worked, but only for a few minutes. Some time later, I did this again, but I checked the machine service manual more carefully and found the explanation: the video output is straight from the ASIC, and is not designed to handle the load of a typical CRT display. The PCW internal display has a buffer, but it is not on the motherboard, for some reason it is on the CRT/power supply board. So I replicated this buffer in my custom computer, and all was well. But then it broke for some other reason.

I mentionned this on a CPC forum and explained that I didn't really want to get into PCW things anymore because of breaking so many of them. But someone really wanted me to do new hardware and sent me yet another one, this time a complete machine with floppy drives and CRT display. I started working on the hardware again, and the machine self-destructed the moment I tried to probe my hardware with an oscilloscope.

So, here I am with a dead motherboard in that machine. The CRT and power suppy are still working fine and I wanted to reuse them. So the idea became to design a new motherboard to fit in there, and interfacing at least with the display, and maybe with the keyboard and the floppy drives.

I started to look for suitable options. I thought it would be nice to do a board I could hand-solder myself, but I don't want to use obsolete, not in production anymore components (let's keep those for fixing existing computers). And it turns out it is not so easy to find modern hardware that can interface with floppy drives and 15KHz monochrome displays, who would have thought?

So I started looking for system-on-modules instead. The idea being that I could find a cheap one, and then design a baseboard for it that would fit inside the PCW.

My choice eventually landed on the Lichee Zero from Sipeed. It is based on the Allwinner V3s chip, which is in itself interesting because it is a QFP package and integrates DRAM in the chip, so you don't need much internal components. Well, it still has a pin spacing that a bit too low, and a bit too many pins, so I probably won't handsolder one myself, but it would be possible.

The Lichee Zero is a board with the CPU and the needed power supply circuits, and a minimal support: one LED, one micro SD slot, one USB port. Everything else is available on castellated holes all around the board with a 1.27mm pitch. It appears the initial idea was a 2.54mm pitch you could use on a breadboard but they included extra pins in half-steps in between.

Anyway, I ordered two of these and proceeded to let them sit on my desk for a few months. This weekend I finally decided to… start a completely different project with them!

The problem with this board is there is really not much you can do with the bare board. You don't have access to a serial port or video output (well, there's a connector for an LCD but I already gave the one compatible LCD I had to someone else who wanted to interface it with an FPGA). And so you really need a baseboard to get started.

So I started thinking about a simple board exposing most of the IOs in a convenient way. I found the schematics from Sipeed for the Lichee Zero and the baseboard they offer for it, and based my work on these. But I added a VGA port, which the original Lichee Zero desgner offered an adapter for, but there are no schematics for that and no one is selling it. I hope my version will be close enough, it is based on a similar adapter designed for the Raspberry Pi.

During my research I found that the Allwinner V3s chip used on this board is now obsolete, and has been replaced by te V3x. I think when I bought the boards, there was not much known about it. Or maybe I didn't know how to search for it. Now there seem to be a community of chinese hackers at the forum working on custom boards using it, and having some success running Linux on it, including apparently a NES emulator with a 6502 CPU core written in hand optimized ARM assembler.

Anyway, in about a day I designed and routed my own baseboard. That would have been simpler if the Lichee Zero had a more standard connector for attaching to a baseboard, but apparently the Lichee Zero Plus, which would be exactly this, is not available. So I had to design my board with a weird shaped cutout in the middle and hope that the Lichee Zero will fit there and align with soldering pads.

I have included the follwing features:

  • On board USB to UART
  • Ethernet port
  • VGA port, with the possibility to connect the DDC lines to the I2C bus on the V3s
  • USB host port (for gamepad or keyboard or...)
  • Audio jack output
  • 4 keys connected to the KEYADC pin (I didn't know what to do with that pin anyways)
  • Expansion connector with easy access to all the remaining pins

The SD card remains on the Lichee Zero module.

In the end the whole project is a bit silly: I started looking for a hand solderable chip, I didn't find one, I picked a system on module with a really strange motherboard attachment scheme, and one that still had a mostly-solderable chip on it even if that didn't matter. And I did not end up designing what I initially intended to do.

We'll see where to go from there. For the PCW project, I have now concluded that a system on module makes sense, because it avoids me doing all the tricky power supply part and also means I don't have to make a huge 4 layer board (it seems hard to route all the needed things for the chip, even the V3s or the V3x, on a 2 layer board). The chip is also strangely balanced, with a 1.2GHz core, but only 64MB of RAM. I don't know if Allwinner will continue doing these chips with a not-so-large amount of DRAM built-in. The new generations of "Lichee" boards from Sipeed appear to use the Allwinner D1, a RISC-V chip with an external 512MB DRAM, and also a somewhat more reasonable form factor (it connects to two side-by-side M2 slots, but of course not using them with the standard things you'd find on an M2 pinout. It also apparently has direct HDMI output, which is kind of nice. But then it's maybe not a thing I can design a baseboard for in just a day.

Probably as usual with these things, by the time I finally order PCBs, and fix all issues in the first batch, the Pi Zero will be all out of stock. Maybe I will never get to build more than 2 machines, and then no one will really write any serious software for it.

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