The tiniest, minimalist 68K computer
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Author:  Plasmo [ Sun Feb 19, 2017 1:37 pm ]
Post subject:  The tiniest, minimalist 68K computer

This may well be the tiniest, minimalist working 68K configuration.

Motorola's 68328, aka DragonBall, was designed in late 1990's for handheld computer such as the Palm. It is an highly integrated processor with a 68EC000 core. Among the peripheral is a LCD controller which was why I bought some in the first place 5+ years ago. To my dismay, I realized I've purchased the BGA version that's too difficult to prototype, but the price is right so I kept them for my 68K CPU collection.
68328_device.jpg [ 175.66 KiB | Viewed 9169 times ]

There is an interesting bootstrap feature on 68328 that supports serial communication with the processor and its peripherals without any external ROM/RAM. Only a dozen signals need to be connected to enable the bootstrap mode. Better yet, the software tool for interfacing in bootstrap mode is still available on the NXP website. So it is conceivable to hand wire a dozen wires to the tiny BGA and earn the bragging right of having power up the tiniest, minimalist 68K system, for what it's worth. :roll:
bootstrap68328scm.jpg [ 282.13 KiB | Viewed 9169 times ]

This is the schematic for enabling the bootstrap mode. All is needed is hookup a 32.768Khz crystal , ground nEMUBKT while resetting and talk to the serial port, that's it. The initial baud rate is 9600, but bbug.exe changes it to 19200, it can even be configure to 115200. bbug.exe is a DOS program with a few basic commands, display memory, modify memory, load program, and go. This is a screen shot of 16 bytes memory in 0xFFFFFF00 being displayed.
bbug_screen_shot.jpg [ 73.75 KiB | Viewed 9169 times ]

Author:  Plasmo [ Sun Feb 19, 2017 1:38 pm ]
Post subject:  Re: The tiniest, minimalist 68K computer

So there you have it, the tiniest, minimalist working 68K computer!
File comment: The tiniest, minimalist 68K computer
DSC_21980219.jpg [ 424 KiB | Viewed 9168 times ]

Author:  Kazu [ Mon Apr 10, 2017 1:50 pm ]
Post subject:  Re: The tiniest, minimalist 68K computer

I have MC68EZ328 CPU.
I was very interested in this project.
I'd like to try it, but I can not find BBUG.exe on the NXP site.
Could you tell me where BBUG.exe is located on the NXP site?
IMG_20170408_111631.jpg [ 104.55 KiB | Viewed 8994 times ]

Author:  Plasmo [ Tue Apr 11, 2017 2:11 am ]
Post subject:  Re: The tiniest, minimalist 68K computer

It was buried deep in MC68EZ328 ADS tools. The name of the zip file is MC68EZ328ADSTOOLS2_0.zip. I went to NXP website and look for it, but somehow I couldn't locate it. It should be there somewhere.

Did you make the board and is it available for purchase?

Author:  Kazu [ Tue Apr 11, 2017 3:18 pm ]
Post subject:  Re: The tiniest, minimalist 68K computer

Thank you for the good information.
I searched the zip file on the nxp site and found the target zip file.
I will try to work next weekend.

MC68EZ328ADSTOOLS2_0.png [ 23.45 KiB | Viewed 8970 times ]

This MC68EZ328 CPU board was made by purchasing CPU chip and QFP100-DIP conversion board and soldering oneself.

Author:  Kazu [ Sat Apr 15, 2017 7:07 am ]
Post subject:  Re: The tiniest, minimalist 68K computer

I was able to run BBUG.EXE by assembling the circuit using the serial USB conversion board.
IMG_20170415_152825.jpg [ 150.51 KiB | Viewed 8940 times ]

ugoita.png [ 34.27 KiB | Viewed 8940 times ]

I devised the connection between CPU and crystal so that wiring length becomes short.
IMG_20170415_154553.jpg [ 108.74 KiB | Viewed 8940 times ]

I will expand this circuit in the future and try to connect DRAM.
I will report it when I can connect successfully.

Author:  Plasmo [ Sat Apr 15, 2017 7:29 pm ]
Post subject:  Re: The tiniest, minimalist 68K computer

Very nice!
I'm surprised the crystal & capacitors network works so far away from the CPU. I think it has a zero-glue interface to DRAM. I'm particularly interested in how well it interface to LCD graphic display. I want to use it as my graphic engine.
Is there a cheap source for 68EZ328? I saw 68VZ328 selling for $5 or less, but not 68EZ328.

Author:  Kazu [ Sat Apr 15, 2017 10:24 pm ]
Post subject:  Re: The tiniest, minimalist 68K computer

I purchased MC68EZ328 with aliexpress. (https://www.aliexpress.com/)
If you search, you will find several shops.
I will use it when purchasing old parts.
Although it may take time to arrive at the product, there is no problem as it is a hobby.

Author:  Kazu [ Fri Apr 28, 2017 6:15 am ]
Post subject:  Re: The tiniest, minimalist 68K computer

I connected the CPU board I made with 256 KB SRAM on the bread board and the operation was unstable.
So I rebuilt the CPU board, wired the power supply and the crystal at the shortest distance, and attached a lot of capacitors.
IMG_20170428_111152.jpg [ 279.63 KiB | Viewed 8773 times ]

When connecting a new CPU board with 256 KB SRAM, the operation became stable.
IMG_20170428_142740.jpg [ 311.95 KiB | Viewed 8773 times ]

BBUG_SRAM.JPG [ 80.93 KiB | Viewed 8773 times ]

I will try to reassemble this circuit from the bread board to the universal board.
Now I can run the 68k program.

Author:  Plasmo [ Sat Apr 29, 2017 4:02 am ]
Post subject:  Re: The tiniest, minimalist 68K computer

Great! So now you can write a bootable monitor software, load and execute out of the RAM. Once that checked out, you can program a set of flash and boot up with it. Do you have plan to make pc board with your design?

Author:  Kazu [ Sat Apr 29, 2017 6:56 am ]
Post subject:  Re: The tiniest, minimalist 68K computer

I chose the memory check program as the first program to run on this system.
The source code of the program was included in the ZIP file.
We modified the source code slightly and created an S format file with the assembler of EASy68K.
memtest1.png [ 54.86 KiB | Viewed 8759 times ]

Since the memory implemented in the system uses two 256Kbits (32K × 8bits) SRAM chips, the memory space is 64Kbytes (0×0000 - 0×ffff).
The program was placed at 0x2000, and it was confirmed that reading and writing to memory can be done normally until 0x4000-0xffff.
memtest2.png [ 45.76 KiB | Viewed 8759 times ]

I would like to create and run a simple monitor program in the same way. Next, I might implement FORTH.
I finally want to run uClinux.
Easy68k is a great tool. Thank you for exciting me.

Author:  legacy [ Tue Jul 11, 2017 1:49 pm ]
Post subject:  Re: The tiniest, minimalist 68K computer

Well done!

Author:  jes246 [ Sat Mar 17, 2018 5:37 am ]
Post subject:  Re: The tiniest, minimalist 68K computer

Can someone give me a link to the bbug.exe program? I was unable to find it on the Nxp site, only user manuals were shown.

I am currently building a small 68vz328 system, and have several Handspring Visor and Sony PDAs which use this processor.

I hope there are still an interest in this bootstrap method.


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