| 68K Multi Board Computer [message #10328] |
Mon, 01 May 2023 20:23  |
c4ooo
Messages: 2
Registered: January 2023
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Junior Member |
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Hello everyone, this is a 68K computer I built last winter. The computer is split into multiple cards so that I could build it up piece by piece while testing some parts on breadboards. The cards use DIN 41612 connectors, but the bus is not VMEBus compatible. The backplane caries most of the signals, however, it does not carry the chip select or interrupt signals. These are carried on the side with jumper wires. This is a bit of an ugly solution, but made it easiser for me to build the computer one card at a time without planning everything from the start.
Backplane
Just a bunch of DIN connectors in parallel and a 7805 regulator. The wires stuck into the port is to connect a video card.
CPU Card
Contains:
- CPU (Currently at 12Mhz, but I want to upgrade to a 16Mhz one)
- Address and low/high byte decoders
- Interrupt circuit (Has 7 edge-triggered channels). Currently only used for PS2 keyboard.
- Power on reset
- Clock & non populated PLL circuit (the Idea being that you could either put a oscillator to run the CPU directly, or use a slower oscillator and then use the PLL to multiply it up to an arbitrary frequency in 1/8mhz steps. However I had some issues prototyping the PLL circuit so I didn't use it. )
Memory Card
Contains:
- 1MB Flash for bootloader/OS
- Upto 2MB SRAM
This card also has buffers to reduce the load on the bus.
IO Card
Contains:
- PS2 Keyboard receiver
- SPI Transmitter for SD card
- RTC (bitbanged)
- GPIO Ports
Some of the GPIO outputs/inputs are used to bitbang the RTC, bitbang data to the keyboard, and activate the CS signal for the SD card. The IO card is probobly the most interesting one so I have included the schematic for it in the attachments.
Video Card
Currently I am using a 80x60 character video card I built to output VGA. However the card was originally built for a z80 computer I had in mind so it only has an 8 bit bus and thus the CPU is only getting half the bandwidth. The next thing for the project will probably be building a video card for this computer. Any ideas for what would be a good fit? I'm thinking 640x480@16-color, but only allow you to set the foreground/background color for a horizontal run of 8 pixels at a time.
Other ideas
Might get around to adding a sound card some time but I'm not really musically inclined so I'm not sure what would work well. Maybe a simple buzzer controlled through one of the IO cards GPIO pins would be good enough? Also DMA would be nice.
Thanks
I want to thank tomstorey for his 68K toolchain, which I have borrowed from, and Xark from the Rosco_m68k project.
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| Re: 68K Multi Board Computer [message #10332 is a reply to message #10331] |
Thu, 04 May 2023 21:58  |
c4ooo
Messages: 2
Registered: January 2023
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Junior Member |
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tingo wrote on Thu, 04 May 2023 09:21Nice! Where did you source the 41612 connectors?
Thanks, you can find the DIN connectors on mouser or digikey. Unfortunately they are about 3 dollars a piece. Here's the ones I used:
https://www.mouser.com/ProductDetail/649-3968114745V1LF
https://www.mouser.com/ProductDetail/649-869396713745ELF
Quote:
Interesting. Always good to see another homebrew 68K project come to life.
What sort of software are you running on your board?
For sound, I'd suggest checking out the YM2419/AY-3-8910 chip as a relatively simple choice. A classic 'chiptune' chip with plenty of example circuits on the web.
-- Rob (robg on the Rosco Discord)
I started writing an "OS" with a file manager and a kernel that could switch between multiple processes. (There's no MMU, so all processes have to be compiled with position independent code in the GCC compiler, which creates a small performance hit). I was thinking though of dropping it in favor a simpler command line interface though. I made a small demo here: https://youtu.be/IDXEfp6GYSY?t=226 I have some of the code on github but it's outdated, I will push the new code some time soon.
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