Tiny68K, a low-cost CP/M-ready 68000 computer
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Author:  Plasmo [ Thu Nov 30, 2017 3:38 pm ]
Post subject:  Tiny68K, a low-cost CP/M-ready 68000 computer

The design goal of Tiny68K is to build a low cost vintage 68000 computer. Vintage parts are fairly inexpensive in the grey market like eBay. On the other hand, they are not highly integrated so large component counts and pc board will run up the cost rapidly. The design challenge is use few integrated components that'll fit in a small pc board. Since 68000 is the given CPU, the design needs to find few low-cost integrated components around it to form a computer system. PC board size is limited to 2-layer 100mm x 100mm because it is 50 cents a piece at that size but price goes up rapidly with more layers and larger size. It should be a 5 volt-only system because voltage translators add up component counts and board space. A modest complexity programmable logic such as Altera's EPM7128 is fairly cheap and can handle all random logic needs. Memory is always the expensive component, especially with the 16-bit wide bus so it needs special attention. The conventional two RAM and two ROM approach is expensive and take up good deal of pc board space. The alternative is one surplus 16-meg DRAM in SIMM72 module and a small, low cost serial EEPROM for boot monitor. It is up to the programmable logic to interface to the DRAM and serial EEPROM. The I/O device is 68681 dual UART. Since serial ports nowadays are mostly USB-to-serial adapter interfacing at TTL level, there are no RS232 transceiver on board. The 16-meg DRAM memory created a different kind of problem: how to fill the memory in a timely manner? Ethernet and high-speed USB interface are possible choices, but I settled on a bus-connected CompactFlash IDE interface. This opens the door for a disk-based operating system like CP/M. What I have after 6 months of tinkering is Tiny68K, a $20 68000 computer with 16-meg memory and run CP/M-68K. EASy68K tool chains is indispensable for the development and debug of monitor software, programming of the EEPROM, and simulation of CP/M 68K.
Tiny68kPCBrev1_F2.jpg [ 485.58 KiB | Viewed 1554 times ]

Author:  profkelly [ Thu Nov 30, 2017 11:06 pm ]
Post subject:  Re: Tiny68K, a low-cost CP/M-ready 68000 computer

Very nice. I think this would make a nice board for use in computer architecture courses.

Author:  Plasmo [ Fri Dec 01, 2017 12:55 am ]
Post subject:  Re: Tiny68K, a low-cost CP/M-ready 68000 computer

Thank you Prof. Kelly! Your comment meant a lot to me. I have used your EASy68K tools extensively in all my 680x0 experimentation this past year. I'm very grateful of your generosity in sharing this wonderful tools with the world. I would be honored to send you a complimentary Tiny68K board.

All my design information and software are public domain but without any warranty expressed or implied. They are published here:
https://www.retrobrewcomputers.org/doku ... bc:tiny68k
You are welcome to take these materials and use them as you see fit. I'll be happy to answer any questions you may have.

I am enthusiastic in sharing my design so early this month I offered 7 Tiny68K kits to hobbyists at $20/kit and was pleasantly surprised that they are all sold quickly. I have plenty of parts for more kits, but I'm out of pc boards. I plan to revise the board design next week and send it off for more pc boards. How many boards do you need for your class?

Author:  profkelly [ Sat Dec 02, 2017 3:24 pm ]
Post subject:  Re: Tiny68K, a low-cost CP/M-ready 68000 computer

I would like to purchase 2 of the kits for my personal use if you decide to offer them for sale again.

Author:  Plasmo [ Sun Dec 03, 2017 12:55 am ]
Post subject:  Re: Tiny68K, a low-cost CP/M-ready 68000 computer

Prof Kelly,
Thank you for your interest. I'll reserve two boards for you and let you know when I'm ready to ship the boards to you. Normally it takes two weeks, but takes longer in December.

Author:  Plasmo [ Sun Dec 17, 2017 1:15 pm ]
Post subject:  Re: Tiny68K, a low-cost CP/M-ready 68000 computer

I received the revised Tiny68K pc boards this week and I built up two boards to check out their functionalities. They both worked as expected. The revised Tiny68K has much more robust CompactFlash interface that works with all the CF cards I have on hand. I also added a non-maskable interrupt button and a real-time clock circuitry. The NMI button serves as warm boot that displays the PC counter and register contents and jump to the beginning of the boot monitor. I have not tested the real-time clock circuitry, waiting for the parts to arrive. CP/M-68K does not need the RTC, but I'm looking ahead to other operating systems that may need the RTC.
The picture shows the revised Tiny68K with the added features annotated.

Tiny68K-rev2_changes copy_F.jpg
Tiny68K-rev2_changes copy_F.jpg [ 548.59 KiB | Viewed 1379 times ]

Author:  mikesmith [ Tue Feb 13, 2018 6:24 am ]
Post subject:  Re: Tiny68K, a low-cost CP/M-ready 68000 computer

If anyone else is playing with this board (it's cute!), I dug out an old m68k emulator I was working on and cleaned it up so that it's easier to write new peripheral drivers.
It emulates the Tiny68k well enough to boot CP/M from an image of the CF card that Plasmo distributes, and has some additional support for ELF executables.

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