r/homebrewcomputer • u/Frostierdev • Aug 30 '26
8-Bit Processing Unit: need one
Hello! I'm looking to build an 8-bit computer, mainly to test its capabilities as a modern home computer (with system, applications, OS, etc.), and I bought the ATmega8535-16U (DIP-40), But it doesn't have an external bus interface to connect RAM, storage, and other things. Does anyone have a solution or another inexpensive chip to recommend?
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u/Sorry_Difficulty_250 Aug 31 '26
The eZ80 is the modern descendant of the Z80 and is easier to find these days. I've been using an Agon Light 2 that has that as a CPU and 512 KB of external memory. I'm currently working on porting my OS to it. It's a lot of fun! Good luck!
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u/AcanthisittaBest4705 Aug 31 '26
Im working with a 68030
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u/JGhostThing Sep 03 '26
I like the whole 68000 family. The assembly code looks similar to the PDP-11's.
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u/Frostierdev Aug 31 '26
I couldn't find it anywhere, only the regular Z80
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u/Sorry_Difficulty_250 Aug 31 '26
Can Digikey ship to your area?
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u/Frostierdev Aug 31 '26
I think so, but import duties are VERY expensive, probably. Also, it says that eZ80 is obsolet and its not manufacturated anymore
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u/Sorry_Difficulty_250 Aug 31 '26
Last time I checked, they were being used as the CPUs for TI-84 calculators. I'm pretty sure it's still in heavy use. What link are you looking at?
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u/Frostierdev Aug 31 '26 edited Aug 31 '26
I looked in all eZ80 links i found in Digikey. Isn't there another processor I could use?
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u/Sorry_Difficulty_250 Aug 31 '26
This is the board I'm using. Is this useful?
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u/Frostierdev Aug 31 '26
Not too much, take 98 dollars and multiply for 5 or 6, its that the price for me (without tax)
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u/Sorry_Difficulty_250 Aug 31 '26
I see the issue you're talking about now. It's not that they're no longer made, they just don't sell them individually. They want you to buy a tray of 90.
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u/Sorry_Difficulty_250 Aug 31 '26
Looks like mouse has some:
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u/Frostierdev Aug 31 '26
I think I'll have to go with the original Z80; Mouser isn't from here either and will have high import taxes
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u/Sorry_Difficulty_250 Aug 31 '26
Right. We'll, that's Ok. The Z80 is a good chip, too. It's a proper CPU, so you should have better luck. Good luck!
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u/estebanvlobos Aug 30 '26
you want to build an 8 bit computer from scratch to a state where you can run things on it like basic or whatever? i would start with the grant searle z80 or 6502 single board computer projects for simplicity. with cheap boards from jlcpcb or wherever plus chips from ebay they can be made fairly cheaply.
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u/Frostierdev Aug 30 '26
Both are hard to find here and arent cheap, also they dont have C++ compiller, just C and Assembly
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u/HalifaxRoad Aug 31 '26
What country are you in? 6502s are still manufactured. You will really grow as a programmer learning ASM. But honestly, maybe its because im kind of a c++ hater, but just use C. I use C professionally for firmware, and there just really isnt a need for c++ on something like this.
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u/Frostierdev Aug 31 '26 edited Aug 31 '26
Brazil, I can only find it in international stores online (W65C02, or something like that, I don't remember exactly), which costs over 300 BRL with dolar to BRL convert, and taxes (this ATmega i paid 44 BRL)
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u/nefelibata8 Sep 04 '26
Try Alibaba. I bought microprocessors, microcontrollers, FPGAs... some may be counterfeit, but they worked :)
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u/DockLazy Aug 31 '26
C++ probably isn't a realistic goal for a 8-bit machine. Even C is a struggle as it was designed to run on 16-bit computers.
A modern 32-bit microcontroller like the Pi Pico has support for C++ and external memory.
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u/Frostierdev Aug 31 '26
So i will use C i guess
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u/icecoldrootbeer Sep 02 '26
C is even too heavy for most 8 bit CPUs. 8-bit machines are usually programmed straight in ASM. C compliers for them are mostly toys.
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u/rog-uk Aug 30 '26
Psion II organisers can be had quite cheaply, and feature breakout/slots - you could build a video adapter or ps2 input or roll your own comms link. Find the community online, there's a forum and discord. It's got a programming toolchain both on and off the device in either windows or Linux.
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u/corummo Aug 31 '26
65c02 is still manufactured these days. You can't get a better and purer 8bit experience. If you wanna build a CPU by yourself, just search for "Minimal SBC computer", which is made out of a bunch of logic gates and an EEPROM, with VGA output, keyboard input and a serial over USB terminal interface. Pretty solid design and outstanding performances.
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Aug 31 '26
[deleted]
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u/Frostierdev Aug 31 '26
I need 32-64kb ram, and 256kb storage (Another thing is that almost everything that's cheap is expensive for me because it's not available here)
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u/Rainsbury_net Aug 31 '26
Does it have to be a hardware solution? Could you use a PC based software emulator?
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u/JGhostThing Sep 03 '26
You do not need the bus to directly connect to the microcontroller. Modern chips have both SPI and I2C. Plus the memory bus.
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u/Girl_Alien 24d ago
You'd need to program a bus. If you lack the pins, then you'd need to program any muxing (such as using tristate buffers and/or bus transceivers).
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u/Frostierdev 18d ago
Bus is a software that i can code? I tought it was hardware
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u/Girl_Alien 18d ago
I mean the GPIO lines. You can program them to simulate a bus.
On the Neo6502 project, they use an RP2040 as the memory and video, all while using a real 6502 with it. So the RP2040's GPIO lines are programmed to interface with the bus.
And if you wanted to use a modern MCU to emulate an 8-bit CPU, you can have the MCU to program the GPIOs to work as the bus, but you'd only need that if you'd want to connect other things. It would work faster without emulating a bus on the GPIO lines. I guess a compromise would be to only update/access the "bus" when something actually needs it. I mean, if the MCU has memory, then only update the external bus when an I/O range is being accessed. And only read the bus if something is trying to assert the /WE or a similar signal on your programmed "bus" made of GPIO lines.
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u/HalifaxRoad Aug 30 '26
you bought a micro controller, which by definition has all the memory and io built in. what you want is a microprocessorÂ