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Designing a very simple 386 motherboard

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First post, by itsemast

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I am not the first person on this forum to design a motherboard, but I want to make one that's very simple and low-cost. The inspiration is mostly coming from Hand386, but I want to strip down all the extras from it (battery, screen, keyboard, graphics card, sound card, USB interface) and only keep the bare minimum.

Here are the specs so far:

  • M6117D SoC (M1386SX CPU + M1217B chipset)
  • TBD DRAM
  • TBD CMOS Flash (BIOS)
  • ISA bus for expansion cards
  • IDE interface for storage media
  • PS/2 interface for keyboard and mouse
  • PC speaker
  • RTC battery
  • GPIO
  • 5V power input

It's somewhat similar to Kharon-386, but instead of tight integration of a lot of components in a really small space, I am going for breaking out all possible interfaces on expansion connectors. I haven't yet decided on what expansions I am going to add first, but a 16550-style UART, a graphics adapter, and Ethernet are definitely on my list of priorities.

I will be publishing my design in Breakout386 project on GitHub. Currently, I am just starting with the schematics, and this is what it looks like:
U4We3sg.png

Let me know what you think! Any thoughts and ideas are appreciated 😀

Reply 1 of 4, by Deunan

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* 350+ ball BGA package
* simple mobo

Pick one.

Reply 2 of 4, by itsemast

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M6117D is 208-lead QFP. With 0.5mm pitch the package size is 28x28 mm, but that's manageable even with a simple 2-layer board. I really want to make it hand-solderable with just a soldering iron, so no BGAs.

Reply 3 of 4, by Deunan

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Clean power delivery, with all the required capacitors, might be tricky on 2-layer mobo but depending on pinout it might be doable with QFP-208. Whether it's going to be simple though you'll learn soon enough I guess.
This being a SoC sure makes things easier if you can route the chip, but if not then consider 386SX (from AMD if you want a low clock) and a bridge from some damaged PC mobo. That won't be as universal since it will require that particular bridge chip but if it's a personal project then it's an option. Other options are 3V3 CPU (including Cyrix/Ti 486SLC) and FPGA as the bridge, or even bigger FPGA and soft-core CPU. I have some varey vague plans to make such FPGA system someday (most likely with actual 3V3 CPU, since I have them and I like the idea better).

Good luck. You'll need it.

Reply 4 of 4, by kingcake

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Deunan wrote on 2024-03-30, 13:14:

Clean power delivery, with all the required capacitors, might be tricky on 2-layer mobo\

Even worse will be EMC/EMI. A 2-layer board like that will radiate like crazy, even if there's room for flood fills.

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